redguardtoo · November 27, 2019 13:29
diff --git a/git-log-L-20,20:README.txt b/git-log-L-20,20:README.txt
 commit 6164972018ba3adbae2db1e7b286e2b811b3e7c2
 Author: Matthieu Moy <[email protected]>
 Date:   Thu Feb 25 09:37:27 2016 +0100

    README.md: add hyperlinks on filenames
    
    Signed-off-by: Matthieu Moy <[email protected]>
    Signed-off-by: Junio C Hamano <[email protected]>

 diff --git a/README.md b/README.md
 --- a/README.md
 +++ b/README.md
 @@ -26,3 +26,1 @@
 -See Documentation/gittutorial.txt to get started, then see
 -Documentation/giteveryday.txt for a useful minimum set of commands, and
 -Documentation/git-commandname.txt for documentation of each command.
 +See [Documentation/gittutorial.txt][] to get started, then see

 commit 673151a9bb56ec97fab66746e3aecef78fddb9b8
 Author: Philip Oakley <[email protected]>
 Date:   Fri Oct 10 22:25:37 2014 +0100

    doc: add 'everyday' to 'git help'
    
    The "Everyday GIT With 20 Commands Or So" is not accessible via the
    Git help system.  Move everyday.txt to giteveryday.txt so that "git
    help everyday" works, and create a new placeholder file everyday.html
    to refer people who follow existing URLs to the updated location.
    
    giteveryday.txt now formats well with AsciiDoc as a man page and
    refreshed content to a more command modern style.
    
    Add 'everyday' to the help --guides list and update git(1) and 5
    other links to giteveryday.
    
    Signed-off-by: Philip Oakley <[email protected]>
    Signed-off-by: Junio C Hamano <[email protected]>

 diff --git a/README b/README
 --- a/README
 +++ b/README
 @@ -29,3 +29,3 @@
 See Documentation/gittutorial.txt to get started, then see
 -Documentation/everyday.txt for a useful minimum set of commands, and
 +Documentation/giteveryday.txt for a useful minimum set of commands, and
 Documentation/git-commandname.txt for documentation of each command.

 commit aa98eb3d6581a123852e4e080011acc3a61bc556
 Author: Christian Couder <[email protected]>
 Date:   Tue Feb 24 21:16:37 2009 +0100

    README: fix path to "gitcvs-migration.txt" and be more consistent
    
    README suggested to look at "Documentation/gittutorial.txt" for the
    tutorial and to use "man git-commandname" for documentation of each
    command.
    
    This was not consistent because the tutorial can also be available with
    "man gittutorial" once git is installed, and the documentation for each
    command can be available at "Documentation/git-commandname.txt" before
    installing git.
    
    This patch tries to make the description more consistent.  It also fixes
    the path to the cvs-migration documentation that changed from
    "Documentation/cvs-migration.txt" to "Documentation/gitcvs-migration.txt".
    
    Signed-off-by: Christian Couder <[email protected]>
    Signed-off-by: Junio C Hamano <[email protected]>

 diff --git a/README b/README
 --- a/README
 +++ b/README
 @@ -27,4 +28,3 @@
 See Documentation/gittutorial.txt to get started, then see
 -Documentation/everyday.txt for a useful minimum set of commands,
 -and "man git-commandname" for documentation of each command.
 -CVS users may also want to read Documentation/cvs-migration.txt.
 +Documentation/everyday.txt for a useful minimum set of commands, and
 +Documentation/git-commandname.txt for documentation of each command.

 commit 8a124b82a03240b10c83085559e5988bc92ea7e2
 Author: Joey Hess <[email protected]>
 Date:   Tue Jan 6 23:23:37 2009 -0500

    README: tutorial.txt is now called gittutorial.txt
    
    Signed-off-by: Joey Hess <[email protected]>
    Signed-off-by: Junio C Hamano <[email protected]>

 diff --git a/README b/README
 --- a/README
 +++ b/README
 @@ -27,4 +27,4 @@
 -See Documentation/tutorial.txt to get started, then see
 +See Documentation/gittutorial.txt to get started, then see
 Documentation/everyday.txt for a useful minimum set of commands,
 and "man git-commandname" for documentation of each command.
 CVS users may also want to read Documentation/cvs-migration.txt.

 commit 556b6600b25713054430b1dcaa731120eefbbd5b
 Author: Nicolas Pitre <[email protected]>
 Date:   Wed Jan 17 13:04:39 2007 -0500

    sanitize content of README file
    
    Current README content is way too esoteric for someone looking at GIT
    for the first time. Instead it should provide a quick summary of what
    GIT is with a few pointers to other resources.
    
    The bulk of the previous README content is moved to
    Documentation/core-intro.txt.
    
    Signed-off-by: Nicolas Pitre <[email protected]>
    Signed-off-by: Junio C Hamano <[email protected]>

 diff --git a/README b/README
 --- a/README
 +++ b/README
 @@ -17,573 +27,4 @@
 -This is a stupid (but extremely fast) directory content manager.  It
 -doesn't do a whole lot, but what it 'does' do is track directory
 -contents efficiently. 
 -
 -There are two object abstractions: the "object database", and the
 -"current directory cache" aka "index".
 -
 -The Object Database
 -~~~~~~~~~~~~~~~~~~~
 -The object database is literally just a content-addressable collection
 -of objects.  All objects are named by their content, which is
 -approximated by the SHA1 hash of the object itself.  Objects may refer
 -to other objects (by referencing their SHA1 hash), and so you can
 -build up a hierarchy of objects.
 -
 -All objects have a statically determined "type" aka "tag", which is
 -determined at object creation time, and which identifies the format of
 -the object (i.e. how it is used, and how it can refer to other
 -objects).  There are currently four different object types: "blob",
 -"tree", "commit" and "tag".
 -
 -A "blob" object cannot refer to any other object, and is, like the type
 -implies, a pure storage object containing some user data.  It is used to
 -actually store the file data, i.e. a blob object is associated with some
 -particular version of some file. 
 -
 -A "tree" object is an object that ties one or more "blob" objects into a
 -directory structure. In addition, a tree object can refer to other tree
 -objects, thus creating a directory hierarchy. 
 -
 -A "commit" object ties such directory hierarchies together into
 -a DAG of revisions - each "commit" is associated with exactly one tree
 -(the directory hierarchy at the time of the commit). In addition, a
 -"commit" refers to one or more "parent" commit objects that describe the
 -history of how we arrived at that directory hierarchy.
 -
 -As a special case, a commit object with no parents is called the "root"
 -object, and is the point of an initial project commit.  Each project
 -must have at least one root, and while you can tie several different
 -root objects together into one project by creating a commit object which
 -has two or more separate roots as its ultimate parents, that's probably
 -just going to confuse people.  So aim for the notion of "one root object
 -per project", even if git itself does not enforce that. 
 -
 -A "tag" object symbolically identifies and can be used to sign other
 -objects. It contains the identifier and type of another object, a
 -symbolic name (of course!) and, optionally, a signature.
 -
 -Regardless of object type, all objects share the following
 -characteristics: they are all deflated with zlib, and have a header
 -that not only specifies their type, but also provides size information
 -about the data in the object.  It's worth noting that the SHA1 hash
 -that is used to name the object is the hash of the original data
 -plus this header, so `sha1sum` 'file' does not match the object name
 -for 'file'.
 -(Historical note: in the dawn of the age of git the hash
 -was the sha1 of the 'compressed' object.)
 -
 -As a result, the general consistency of an object can always be tested
 -independently of the contents or the type of the object: all objects can
 -be validated by verifying that (a) their hashes match the content of the
 -file and (b) the object successfully inflates to a stream of bytes that
 -forms a sequence of <ascii type without space> + <space> + <ascii decimal
 -size> + <byte\0> + <binary object data>. 
 -
 -The structured objects can further have their structure and
 -connectivity to other objects verified. This is generally done with
 -the `git-fsck-objects` program, which generates a full dependency graph
 -of all objects, and verifies their internal consistency (in addition
 -to just verifying their superficial consistency through the hash).
 -
 -The object types in some more detail:
 -
 -Blob Object
 -~~~~~~~~~~~
 -A "blob" object is nothing but a binary blob of data, and doesn't
 -refer to anything else.  There is no signature or any other
 -verification of the data, so while the object is consistent (it 'is'
 -indexed by its sha1 hash, so the data itself is certainly correct), it
 -has absolutely no other attributes.  No name associations, no
 -permissions.  It is purely a blob of data (i.e. normally "file
 -contents").
 -
 -In particular, since the blob is entirely defined by its data, if two
 -files in a directory tree (or in multiple different versions of the
 -repository) have the same contents, they will share the same blob
 -object. The object is totally independent of its location in the
 -directory tree, and renaming a file does not change the object that
 -file is associated with in any way.
 -
 -A blob is typically created when gitlink:git-update-index[1]
 -is run, and its data can be accessed by gitlink:git-cat-file[1].
 -
 -Tree Object
 -~~~~~~~~~~~
 -The next hierarchical object type is the "tree" object.  A tree object
 -is a list of mode/name/blob data, sorted by name.  Alternatively, the
 -mode data may specify a directory mode, in which case instead of
 -naming a blob, that name is associated with another TREE object.
 -
 -Like the "blob" object, a tree object is uniquely determined by the
 -set contents, and so two separate but identical trees will always
 -share the exact same object. This is true at all levels, i.e. it's
 -true for a "leaf" tree (which does not refer to any other trees, only
 -blobs) as well as for a whole subdirectory.
 -
 -For that reason a "tree" object is just a pure data abstraction: it
 -has no history, no signatures, no verification of validity, except
 -that since the contents are again protected by the hash itself, we can
 -trust that the tree is immutable and its contents never change.
 -
 -So you can trust the contents of a tree to be valid, the same way you
 -can trust the contents of a blob, but you don't know where those
 -contents 'came' from.
 -
 -Side note on trees: since a "tree" object is a sorted list of
 -"filename+content", you can create a diff between two trees without
 -actually having to unpack two trees.  Just ignore all common parts,
 -and your diff will look right.  In other words, you can effectively
 -(and efficiently) tell the difference between any two random trees by
 -O(n) where "n" is the size of the difference, rather than the size of
 -the tree.
 -
 -Side note 2 on trees: since the name of a "blob" depends entirely and
 -exclusively on its contents (i.e. there are no names or permissions
 -involved), you can see trivial renames or permission changes by
 -noticing that the blob stayed the same.  However, renames with data
 -changes need a smarter "diff" implementation.
 -
 -A tree is created with gitlink:git-write-tree[1] and
 -its data can be accessed by gitlink:git-ls-tree[1].
 -Two trees can be compared with gitlink:git-diff-tree[1].
 -
 -Commit Object
 -~~~~~~~~~~~~~
 -The "commit" object is an object that introduces the notion of
 -history into the picture.  In contrast to the other objects, it
 -doesn't just describe the physical state of a tree, it describes how
 -we got there, and why.
 -
 -A "commit" is defined by the tree-object that it results in, the
 -parent commits (zero, one or more) that led up to that point, and a
 -comment on what happened.  Again, a commit is not trusted per se:
 -the contents are well-defined and "safe" due to the cryptographically
 -strong signatures at all levels, but there is no reason to believe
 -that the tree is "good" or that the merge information makes sense.
 -The parents do not have to actually have any relationship with the
 -result, for example.
 -
 -Note on commits: unlike real SCM's, commits do not contain
 -rename information or file mode change information.  All of that is
 -implicit in the trees involved (the result tree, and the result trees
 -of the parents), and describing that makes no sense in this idiotic
 -file manager.
 -
 -A commit is created with gitlink:git-commit-tree[1] and
 -its data can be accessed by gitlink:git-cat-file[1].
 -
 -Trust
 -~~~~~
 -An aside on the notion of "trust". Trust is really outside the scope
 -of "git", but it's worth noting a few things.  First off, since
 -everything is hashed with SHA1, you 'can' trust that an object is
 -intact and has not been messed with by external sources.  So the name
 -of an object uniquely identifies a known state - just not a state that
 -you may want to trust.
 -
 -Furthermore, since the SHA1 signature of a commit refers to the
 -SHA1 signatures of the tree it is associated with and the signatures
 -of the parent, a single named commit specifies uniquely a whole set
 -of history, with full contents.  You can't later fake any step of the
 -way once you have the name of a commit.
 -
 -So to introduce some real trust in the system, the only thing you need
 -to do is to digitally sign just 'one' special note, which includes the
 -name of a top-level commit.  Your digital signature shows others
 -that you trust that commit, and the immutability of the history of
 -commits tells others that they can trust the whole history.
 -
 -In other words, you can easily validate a whole archive by just
 -sending out a single email that tells the people the name (SHA1 hash)
 -of the top commit, and digitally sign that email using something
 -like GPG/PGP.
 -
 -To assist in this, git also provides the tag object...
 -
 -Tag Object
 -~~~~~~~~~~
 -Git provides the "tag" object to simplify creating, managing and
 -exchanging symbolic and signed tokens.  The "tag" object at its
 -simplest simply symbolically identifies another object by containing
 -the sha1, type and symbolic name.
 -
 -However it can optionally contain additional signature information
 -(which git doesn't care about as long as there's less than 8k of
 -it). This can then be verified externally to git.
 -
 -Note that despite the tag features, "git" itself only handles content
 -integrity; the trust framework (and signature provision and
 -verification) has to come from outside.
 -
 -A tag is created with gitlink:git-mktag[1],
 -its data can be accessed by gitlink:git-cat-file[1],
 -and the signature can be verified by
 -gitlink:git-verify-tag[1].
 -
 -
 -The "index" aka "Current Directory Cache"
 ------------------------------------------
 -The index is a simple binary file, which contains an efficient
 -representation of a virtual directory content at some random time.  It
 -does so by a simple array that associates a set of names, dates,
 -permissions and content (aka "blob") objects together.  The cache is
 -always kept ordered by name, and names are unique (with a few very
 -specific rules) at any point in time, but the cache has no long-term
 -meaning, and can be partially updated at any time.
 -
 -In particular, the index certainly does not need to be consistent with
 -the current directory contents (in fact, most operations will depend on
 -different ways to make the index 'not' be consistent with the directory
 -hierarchy), but it has three very important attributes:
 -
 -'(a) it can re-generate the full state it caches (not just the
 -directory structure: it contains pointers to the "blob" objects so
 -that it can regenerate the data too)'
 -
 -As a special case, there is a clear and unambiguous one-way mapping
 -from a current directory cache to a "tree object", which can be
 -efficiently created from just the current directory cache without
 -actually looking at any other data.  So a directory cache at any one
 -time uniquely specifies one and only one "tree" object (but has
 -additional data to make it easy to match up that tree object with what
 -has happened in the directory)
 -
 -'(b) it has efficient methods for finding inconsistencies between that
 -cached state ("tree object waiting to be instantiated") and the
 -current state.'
 -
 -'(c) it can additionally efficiently represent information about merge
 -conflicts between different tree objects, allowing each pathname to be
 -associated with sufficient information about the trees involved that
 -you can create a three-way merge between them.'
 -
 -Those are the three ONLY things that the directory cache does.  It's a
 -cache, and the normal operation is to re-generate it completely from a
 -known tree object, or update/compare it with a live tree that is being
 -developed.  If you blow the directory cache away entirely, you generally
 -haven't lost any information as long as you have the name of the tree
 -that it described. 
 -
 -At the same time, the index is at the same time also the
 -staging area for creating new trees, and creating a new tree always
 -involves a controlled modification of the index file.  In particular,
 -the index file can have the representation of an intermediate tree that
 -has not yet been instantiated.  So the index can be thought of as a
 -write-back cache, which can contain dirty information that has not yet
 -been written back to the backing store.
 -
 -
 -
 -The Workflow
 -------------
 -Generally, all "git" operations work on the index file. Some operations
 -work *purely* on the index file (showing the current state of the
 -index), but most operations move data to and from the index file. Either
 -from the database or from the working directory. Thus there are four
 -main combinations: 
 -
 -1) working directory -> index
 -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 -
 -You update the index with information from the working directory with
 -the gitlink:git-update-index[1] command.  You
 -generally update the index information by just specifying the filename
 -you want to update, like so:
 -
 -	git-update-index filename
 -
 -but to avoid common mistakes with filename globbing etc, the command
 -will not normally add totally new entries or remove old entries,
 -i.e. it will normally just update existing cache entries.
 -
 -To tell git that yes, you really do realize that certain files no
 -longer exist, or that new files should be added, you
 -should use the `--remove` and `--add` flags respectively.
 -
 -NOTE! A `--remove` flag does 'not' mean that subsequent filenames will
 -necessarily be removed: if the files still exist in your directory
 -structure, the index will be updated with their new status, not
 -removed. The only thing `--remove` means is that update-cache will be
 -considering a removed file to be a valid thing, and if the file really
 -does not exist any more, it will update the index accordingly.
 -
 -As a special case, you can also do `git-update-index --refresh`, which
 -will refresh the "stat" information of each index to match the current
 -stat information. It will 'not' update the object status itself, and
 -it will only update the fields that are used to quickly test whether
 -an object still matches its old backing store object.
 -
 -2) index -> object database
 -~~~~~~~~~~~~~~~~~~~~~~~~~~~
 -
 -You write your current index file to a "tree" object with the program
 -
 -	git-write-tree
 -
 -that doesn't come with any options - it will just write out the
 -current index into the set of tree objects that describe that state,
 -and it will return the name of the resulting top-level tree. You can
 -use that tree to re-generate the index at any time by going in the
 -other direction:
 -
 -3) object database -> index
 -~~~~~~~~~~~~~~~~~~~~~~~~~~~
 -
 -You read a "tree" file from the object database, and use that to
 -populate (and overwrite - don't do this if your index contains any
 -unsaved state that you might want to restore later!) your current
 -index.  Normal operation is just
 -
 -		git-read-tree <sha1 of tree>
 -
 -and your index file will now be equivalent to the tree that you saved
 -earlier. However, that is only your 'index' file: your working
 -directory contents have not been modified.
 -
 -4) index -> working directory
 -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 -
 -You update your working directory from the index by "checking out"
 -files. This is not a very common operation, since normally you'd just
 -keep your files updated, and rather than write to your working
 -directory, you'd tell the index files about the changes in your
 -working directory (i.e. `git-update-index`).
 -
 -However, if you decide to jump to a new version, or check out somebody
 -else's version, or just restore a previous tree, you'd populate your
 -index file with read-tree, and then you need to check out the result
 -with
 -
 -		git-checkout-index filename
 -
 -or, if you want to check out all of the index, use `-a`.
 -
 -NOTE! git-checkout-index normally refuses to overwrite old files, so
 -if you have an old version of the tree already checked out, you will
 -need to use the "-f" flag ('before' the "-a" flag or the filename) to
 -'force' the checkout.
 -
 -
 -Finally, there are a few odds and ends which are not purely moving
 -from one representation to the other:
 -
 -5) Tying it all together
 -~~~~~~~~~~~~~~~~~~~~~~~~
 -To commit a tree you have instantiated with "git-write-tree", you'd
 -create a "commit" object that refers to that tree and the history
 -behind it - most notably the "parent" commits that preceded it in
 -history.
 -
 -Normally a "commit" has one parent: the previous state of the tree
 -before a certain change was made. However, sometimes it can have two
 -or more parent commits, in which case we call it a "merge", due to the
 -fact that such a commit brings together ("merges") two or more
 -previous states represented by other commits.
 -
 -In other words, while a "tree" represents a particular directory state
 -of a working directory, a "commit" represents that state in "time",
 -and explains how we got there.
 -
 -You create a commit object by giving it the tree that describes the
 -state at the time of the commit, and a list of parents:
 -
 -	git-commit-tree <tree> -p <parent> [-p <parent2> ..]
 -
 -and then giving the reason for the commit on stdin (either through
 -redirection from a pipe or file, or by just typing it at the tty).
 -
 -git-commit-tree will return the name of the object that represents
 -that commit, and you should save it away for later use. Normally,
 -you'd commit a new `HEAD` state, and while git doesn't care where you
 -save the note about that state, in practice we tend to just write the
 -result to the file pointed at by `.git/HEAD`, so that we can always see
 -what the last committed state was.
 -
 -Here is an ASCII art by Jon Loeliger that illustrates how
 -various pieces fit together.
 -
 -------------
 -
 -                     commit-tree
 -                      commit obj
 -                       +----+
 -                       |    |
 -                       |    |
 -                       V    V
 -                    +-----------+
 -                    | Object DB |
 -                    |  Backing  |
 -                    |   Store   |
 -                    +-----------+
 -                       ^
 -           write-tree  |     |
 -             tree obj  |     |
 -                       |     |  read-tree
 -                       |     |  tree obj
 -                             V
 -                    +-----------+
 -                    |   Index   |
 -                    |  "cache"  |
 -                    +-----------+
 -         update-index  ^
 -             blob obj  |     |
 -                       |     |
 -    checkout-index -u  |     |  checkout-index
 -             stat      |     |  blob obj
 -                             V
 -                    +-----------+
 -                    |  Working  |
 -                    | Directory |
 -                    +-----------+
 -
 -------------
 -
 -
 -6) Examining the data
 -~~~~~~~~~~~~~~~~~~~~~
 -
 -You can examine the data represented in the object database and the
 -index with various helper tools. For every object, you can use
 -gitlink:git-cat-file[1] to examine details about the
 -object:
 -
 -		git-cat-file -t <objectname>
 -
 -shows the type of the object, and once you have the type (which is
 -usually implicit in where you find the object), you can use
 -
 -		git-cat-file blob|tree|commit|tag <objectname>
 -
 -to show its contents. NOTE! Trees have binary content, and as a result
 -there is a special helper for showing that content, called
 -`git-ls-tree`, which turns the binary content into a more easily
 -readable form.
 -
 -It's especially instructive to look at "commit" objects, since those
 -tend to be small and fairly self-explanatory. In particular, if you
 -follow the convention of having the top commit name in `.git/HEAD`,
 -you can do
 -
 -		git-cat-file commit HEAD
 -
 -to see what the top commit was.
 -
 -7) Merging multiple trees
 -~~~~~~~~~~~~~~~~~~~~~~~~~
 -
 -Git helps you do a three-way merge, which you can expand to n-way by
 -repeating the merge procedure arbitrary times until you finally
 -"commit" the state.  The normal situation is that you'd only do one
 -three-way merge (two parents), and commit it, but if you like to, you
 -can do multiple parents in one go.
 -
 -To do a three-way merge, you need the two sets of "commit" objects
 -that you want to merge, use those to find the closest common parent (a
 -third "commit" object), and then use those commit objects to find the
 -state of the directory ("tree" object) at these points.
 -
 -To get the "base" for the merge, you first look up the common parent
 -of two commits with
 -
 -		git-merge-base <commit1> <commit2>
 -
 -which will return you the commit they are both based on.  You should
 -now look up the "tree" objects of those commits, which you can easily
 -do with (for example)
 -
 -		git-cat-file commit <commitname> | head -1
 -
 -since the tree object information is always the first line in a commit
 -object.
 -
 -Once you know the three trees you are going to merge (the one
 -"original" tree, aka the common case, and the two "result" trees, aka
 -the branches you want to merge), you do a "merge" read into the
 -index. This will complain if it has to throw away your old index contents, so you should
 -make sure that you've committed those - in fact you would normally
 -always do a merge against your last commit (which should thus match
 -what you have in your current index anyway).
 -
 -To do the merge, do
 -
 -		git-read-tree -m -u <origtree> <yourtree> <targettree>
 -
 -which will do all trivial merge operations for you directly in the
 -index file, and you can just write the result out with
 -`git-write-tree`.
 -
 -Historical note.  We did not have `-u` facility when this
 -section was first written, so we used to warn that
 -the merge is done in the index file, not in your
 -working tree, and your working tree will not match your
 -index after this step.
 -This is no longer true.  The above command, thanks to `-u`
 -option, updates your working tree with the merge results for
 -paths that have been trivially merged.
 -
 -
 -8) Merging multiple trees, continued
 -~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 -
 -Sadly, many merges aren't trivial. If there are files that have
 -been added.moved or removed, or if both branches have modified the
 -same file, you will be left with an index tree that contains "merge
 -entries" in it. Such an index tree can 'NOT' be written out to a tree
 -object, and you will have to resolve any such merge clashes using
 -other tools before you can write out the result.
 -
 -You can examine such index state with `git-ls-files --unmerged`
 -command.  An example:
 -
 -------------------------------------------------
 -$ git-read-tree -m $orig HEAD $target
 -$ git-ls-files --unmerged
 -100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1	hello.c
 -100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2	hello.c
 -100644 cc44c73eb783565da5831b4d820c962954019b69 3	hello.c
 -------------------------------------------------
 -
 -Each line of the `git-ls-files --unmerged` output begins with
 -the blob mode bits, blob SHA1, 'stage number', and the
 -filename.  The 'stage number' is git's way to say which tree it
 -came from: stage 1 corresponds to `$orig` tree, stage 2 `HEAD`
 -tree, and stage3 `$target` tree.
 -
 -Earlier we said that trivial merges are done inside
 -`git-read-tree -m`.  For example, if the file did not change
 -from `$orig` to `HEAD` nor `$target`, or if the file changed
 -from `$orig` to `HEAD` and `$orig` to `$target` the same way,
 -obviously the final outcome is what is in `HEAD`.  What the
 -above example shows is that file `hello.c` was changed from
 -`$orig` to `HEAD` and `$orig` to `$target` in a different way.
 -You could resolve this by running your favorite 3-way merge
 -program, e.g.  `diff3` or `merge`, on the blob objects from
 -these three stages yourself, like this:
 -
 -------------------------------------------------
 -$ git-cat-file blob 263414f... >hello.c~1
 -$ git-cat-file blob 06fa6a2... >hello.c~2
 -$ git-cat-file blob cc44c73... >hello.c~3
 -$ merge hello.c~2 hello.c~1 hello.c~3
 -------------------------------------------------
 -
 -This would leave the merge result in `hello.c~2` file, along
 -with conflict markers if there are conflicts.  After verifying
 -the merge result makes sense, you can tell git what the final
 -merge result for this file is by:
 -
 -	mv -f hello.c~2 hello.c
 -	git-update-index hello.c
 -
 -When a path is in unmerged state, running `git-update-index` for
 -that path tells git to mark the path resolved.
 -
 -The above is the description of a git merge at the lowest level,
 -to help you understand what conceptually happens under the hood.
 -In practice, nobody, not even git itself, uses three `git-cat-file`
 -for this.  There is `git-merge-index` program that extracts the
 -stages to temporary files and calls a "merge" script on it:
 -
 -	git-merge-index git-merge-one-file hello.c
 -
 -and that is what higher level `git resolve` is implemented with.
 +See Documentation/tutorial.txt to get started, then see
 +Documentation/everyday.txt for a useful minimum set of commands,
 +and "man git-commandname" for documentation of each command.
 +CVS users may also want to read Documentation/cvs-migration.txt.

 commit 2fa090b6c145db9f6219a037c773fb63fe727019
 Author: Junio C Hamano <[email protected]>
 Date:   Wed Dec 7 16:05:21 2005 -0800

    Documentation: git.html/git.7
    
    Finish each sentence with a full stop.
    
    Instead of saying 'directory index' 'directory cache' etc,
    consistently say 'index'.
    
    Signed-off-by: Junio C Hamano <[email protected]>

 diff --git a/README b/README
 --- a/README
 +++ b/README
 @@ -17,570 +17,573 @@
 This is a stupid (but extremely fast) directory content manager.  It
 doesn't do a whole lot, but what it 'does' do is track directory
 contents efficiently. 
 
 There are two object abstractions: the "object database", and the
 "current directory cache" aka "index".
 
 The Object Database
 ~~~~~~~~~~~~~~~~~~~
 The object database is literally just a content-addressable collection
 of objects.  All objects are named by their content, which is
 approximated by the SHA1 hash of the object itself.  Objects may refer
 to other objects (by referencing their SHA1 hash), and so you can
 build up a hierarchy of objects.
 
 All objects have a statically determined "type" aka "tag", which is
 determined at object creation time, and which identifies the format of
 the object (i.e. how it is used, and how it can refer to other
 objects).  There are currently four different object types: "blob",
 "tree", "commit" and "tag".
 
 -A "blob" object cannot refer to any other object, and is, like the tag
 +A "blob" object cannot refer to any other object, and is, like the type
 implies, a pure storage object containing some user data.  It is used to
 actually store the file data, i.e. a blob object is associated with some
 particular version of some file. 
 
 A "tree" object is an object that ties one or more "blob" objects into a
 directory structure. In addition, a tree object can refer to other tree
 objects, thus creating a directory hierarchy. 
 
 A "commit" object ties such directory hierarchies together into
 a DAG of revisions - each "commit" is associated with exactly one tree
 (the directory hierarchy at the time of the commit). In addition, a
 "commit" refers to one or more "parent" commit objects that describe the
 history of how we arrived at that directory hierarchy.
 
 As a special case, a commit object with no parents is called the "root"
 object, and is the point of an initial project commit.  Each project
 must have at least one root, and while you can tie several different
 root objects together into one project by creating a commit object which
 has two or more separate roots as its ultimate parents, that's probably
 just going to confuse people.  So aim for the notion of "one root object
 per project", even if git itself does not enforce that. 
 
 A "tag" object symbolically identifies and can be used to sign other
 objects. It contains the identifier and type of another object, a
 symbolic name (of course!) and, optionally, a signature.
 
 Regardless of object type, all objects share the following
 characteristics: they are all deflated with zlib, and have a header
 -that not only specifies their tag, but also provides size information
 +that not only specifies their type, but also provides size information
 about the data in the object.  It's worth noting that the SHA1 hash
 that is used to name the object is the hash of the original data
 plus this header, so `sha1sum` 'file' does not match the object name
 for 'file'.
 (Historical note: in the dawn of the age of git the hash
 was the sha1 of the 'compressed' object.)
 
 As a result, the general consistency of an object can always be tested
 independently of the contents or the type of the object: all objects can
 be validated by verifying that (a) their hashes match the content of the
 file and (b) the object successfully inflates to a stream of bytes that
 -forms a sequence of <ascii tag without space> + <space> + <ascii decimal
 +forms a sequence of <ascii type without space> + <space> + <ascii decimal
 size> + <byte\0> + <binary object data>. 
 
 The structured objects can further have their structure and
 connectivity to other objects verified. This is generally done with
 the `git-fsck-objects` program, which generates a full dependency graph
 of all objects, and verifies their internal consistency (in addition
 to just verifying their superficial consistency through the hash).
 
 The object types in some more detail:
 
 Blob Object
 ~~~~~~~~~~~
 A "blob" object is nothing but a binary blob of data, and doesn't
 refer to anything else.  There is no signature or any other
 verification of the data, so while the object is consistent (it 'is'
 indexed by its sha1 hash, so the data itself is certainly correct), it
 has absolutely no other attributes.  No name associations, no
 permissions.  It is purely a blob of data (i.e. normally "file
 contents").
 
 In particular, since the blob is entirely defined by its data, if two
 files in a directory tree (or in multiple different versions of the
 repository) have the same contents, they will share the same blob
 object. The object is totally independent of its location in the
 directory tree, and renaming a file does not change the object that
 file is associated with in any way.
 
 A blob is typically created when gitlink:git-update-index[1]
 is run, and its data can be accessed by gitlink:git-cat-file[1].
 
 Tree Object
 ~~~~~~~~~~~
 The next hierarchical object type is the "tree" object.  A tree object
 is a list of mode/name/blob data, sorted by name.  Alternatively, the
 mode data may specify a directory mode, in which case instead of
 naming a blob, that name is associated with another TREE object.
 
 Like the "blob" object, a tree object is uniquely determined by the
 set contents, and so two separate but identical trees will always
 share the exact same object. This is true at all levels, i.e. it's
 true for a "leaf" tree (which does not refer to any other trees, only
 blobs) as well as for a whole subdirectory.
 
 For that reason a "tree" object is just a pure data abstraction: it
 has no history, no signatures, no verification of validity, except
 that since the contents are again protected by the hash itself, we can
 trust that the tree is immutable and its contents never change.
 
 So you can trust the contents of a tree to be valid, the same way you
 can trust the contents of a blob, but you don't know where those
 contents 'came' from.
 
 Side note on trees: since a "tree" object is a sorted list of
 "filename+content", you can create a diff between two trees without
 actually having to unpack two trees.  Just ignore all common parts,
 and your diff will look right.  In other words, you can effectively
 (and efficiently) tell the difference between any two random trees by
 O(n) where "n" is the size of the difference, rather than the size of
 the tree.
 
 Side note 2 on trees: since the name of a "blob" depends entirely and
 exclusively on its contents (i.e. there are no names or permissions
 involved), you can see trivial renames or permission changes by
 noticing that the blob stayed the same.  However, renames with data
 changes need a smarter "diff" implementation.
 
 A tree is created with gitlink:git-write-tree[1] and
 its data can be accessed by gitlink:git-ls-tree[1].
 Two trees can be compared with gitlink:git-diff-tree[1].
 
 Commit Object
 ~~~~~~~~~~~~~
 The "commit" object is an object that introduces the notion of
 history into the picture.  In contrast to the other objects, it
 doesn't just describe the physical state of a tree, it describes how
 we got there, and why.
 
 A "commit" is defined by the tree-object that it results in, the
 parent commits (zero, one or more) that led up to that point, and a
 comment on what happened.  Again, a commit is not trusted per se:
 the contents are well-defined and "safe" due to the cryptographically
 strong signatures at all levels, but there is no reason to believe
 that the tree is "good" or that the merge information makes sense.
 The parents do not have to actually have any relationship with the
 result, for example.
 
 Note on commits: unlike real SCM's, commits do not contain
 rename information or file mode change information.  All of that is
 implicit in the trees involved (the result tree, and the result trees
 of the parents), and describing that makes no sense in this idiotic
 file manager.
 
 A commit is created with gitlink:git-commit-tree[1] and
 its data can be accessed by gitlink:git-cat-file[1].
 
 Trust
 ~~~~~
 An aside on the notion of "trust". Trust is really outside the scope
 of "git", but it's worth noting a few things.  First off, since
 everything is hashed with SHA1, you 'can' trust that an object is
 intact and has not been messed with by external sources.  So the name
 of an object uniquely identifies a known state - just not a state that
 you may want to trust.
 
 Furthermore, since the SHA1 signature of a commit refers to the
 SHA1 signatures of the tree it is associated with and the signatures
 of the parent, a single named commit specifies uniquely a whole set
 of history, with full contents.  You can't later fake any step of the
 way once you have the name of a commit.
 
 So to introduce some real trust in the system, the only thing you need
 to do is to digitally sign just 'one' special note, which includes the
 name of a top-level commit.  Your digital signature shows others
 that you trust that commit, and the immutability of the history of
 commits tells others that they can trust the whole history.
 
 In other words, you can easily validate a whole archive by just
 sending out a single email that tells the people the name (SHA1 hash)
 of the top commit, and digitally sign that email using something
 like GPG/PGP.
 
 To assist in this, git also provides the tag object...
 
 Tag Object
 ~~~~~~~~~~
 Git provides the "tag" object to simplify creating, managing and
 exchanging symbolic and signed tokens.  The "tag" object at its
 simplest simply symbolically identifies another object by containing
 the sha1, type and symbolic name.
 
 However it can optionally contain additional signature information
 (which git doesn't care about as long as there's less than 8k of
 it). This can then be verified externally to git.
 
 Note that despite the tag features, "git" itself only handles content
 integrity; the trust framework (and signature provision and
 verification) has to come from outside.
 
 A tag is created with gitlink:git-mktag[1],
 its data can be accessed by gitlink:git-cat-file[1],
 and the signature can be verified by
 gitlink:git-verify-tag[1].
 
 
 The "index" aka "Current Directory Cache"
 -----------------------------------------
 The index is a simple binary file, which contains an efficient
 representation of a virtual directory content at some random time.  It
 does so by a simple array that associates a set of names, dates,
 permissions and content (aka "blob") objects together.  The cache is
 always kept ordered by name, and names are unique (with a few very
 specific rules) at any point in time, but the cache has no long-term
 meaning, and can be partially updated at any time.
 
 In particular, the index certainly does not need to be consistent with
 the current directory contents (in fact, most operations will depend on
 different ways to make the index 'not' be consistent with the directory
 hierarchy), but it has three very important attributes:
 
 '(a) it can re-generate the full state it caches (not just the
 directory structure: it contains pointers to the "blob" objects so
 that it can regenerate the data too)'
 
 As a special case, there is a clear and unambiguous one-way mapping
 from a current directory cache to a "tree object", which can be
 efficiently created from just the current directory cache without
 actually looking at any other data.  So a directory cache at any one
 time uniquely specifies one and only one "tree" object (but has
 additional data to make it easy to match up that tree object with what
 has happened in the directory)
 
 '(b) it has efficient methods for finding inconsistencies between that
 cached state ("tree object waiting to be instantiated") and the
 current state.'
 
 '(c) it can additionally efficiently represent information about merge
 conflicts between different tree objects, allowing each pathname to be
 associated with sufficient information about the trees involved that
 you can create a three-way merge between them.'
 
 Those are the three ONLY things that the directory cache does.  It's a
 cache, and the normal operation is to re-generate it completely from a
 known tree object, or update/compare it with a live tree that is being
 developed.  If you blow the directory cache away entirely, you generally
 haven't lost any information as long as you have the name of the tree
 that it described. 
 
 At the same time, the index is at the same time also the
 staging area for creating new trees, and creating a new tree always
 involves a controlled modification of the index file.  In particular,
 the index file can have the representation of an intermediate tree that
 has not yet been instantiated.  So the index can be thought of as a
 write-back cache, which can contain dirty information that has not yet
 been written back to the backing store.
 
 
 
 The Workflow
 ------------
 Generally, all "git" operations work on the index file. Some operations
 work *purely* on the index file (showing the current state of the
 index), but most operations move data to and from the index file. Either
 from the database or from the working directory. Thus there are four
 main combinations: 
 
 1) working directory -> index
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You update the index with information from the working directory with
 the gitlink:git-update-index[1] command.  You
 generally update the index information by just specifying the filename
 you want to update, like so:
 
 	git-update-index filename
 
 but to avoid common mistakes with filename globbing etc, the command
 will not normally add totally new entries or remove old entries,
 i.e. it will normally just update existing cache entries.
 
 To tell git that yes, you really do realize that certain files no
 -longer exist in the archive, or that new files should be added, you
 +longer exist, or that new files should be added, you
 should use the `--remove` and `--add` flags respectively.
 
 NOTE! A `--remove` flag does 'not' mean that subsequent filenames will
 necessarily be removed: if the files still exist in your directory
 structure, the index will be updated with their new status, not
 removed. The only thing `--remove` means is that update-cache will be
 considering a removed file to be a valid thing, and if the file really
 does not exist any more, it will update the index accordingly.
 
 As a special case, you can also do `git-update-index --refresh`, which
 will refresh the "stat" information of each index to match the current
 stat information. It will 'not' update the object status itself, and
 it will only update the fields that are used to quickly test whether
 an object still matches its old backing store object.
 
 2) index -> object database
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You write your current index file to a "tree" object with the program
 
 	git-write-tree
 
 that doesn't come with any options - it will just write out the
 current index into the set of tree objects that describe that state,
 and it will return the name of the resulting top-level tree. You can
 use that tree to re-generate the index at any time by going in the
 other direction:
 
 3) object database -> index
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You read a "tree" file from the object database, and use that to
 populate (and overwrite - don't do this if your index contains any
 unsaved state that you might want to restore later!) your current
 index.  Normal operation is just
 
 		git-read-tree <sha1 of tree>
 
 and your index file will now be equivalent to the tree that you saved
 earlier. However, that is only your 'index' file: your working
 directory contents have not been modified.
 
 4) index -> working directory
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You update your working directory from the index by "checking out"
 files. This is not a very common operation, since normally you'd just
 keep your files updated, and rather than write to your working
 directory, you'd tell the index files about the changes in your
 working directory (i.e. `git-update-index`).
 
 However, if you decide to jump to a new version, or check out somebody
 else's version, or just restore a previous tree, you'd populate your
 index file with read-tree, and then you need to check out the result
 with
 
 		git-checkout-index filename
 
 or, if you want to check out all of the index, use `-a`.
 
 NOTE! git-checkout-index normally refuses to overwrite old files, so
 if you have an old version of the tree already checked out, you will
 need to use the "-f" flag ('before' the "-a" flag or the filename) to
 'force' the checkout.
 
 
 Finally, there are a few odds and ends which are not purely moving
 from one representation to the other:
 
 5) Tying it all together
 ~~~~~~~~~~~~~~~~~~~~~~~~
 To commit a tree you have instantiated with "git-write-tree", you'd
 create a "commit" object that refers to that tree and the history
 behind it - most notably the "parent" commits that preceded it in
 history.
 
 Normally a "commit" has one parent: the previous state of the tree
 before a certain change was made. However, sometimes it can have two
 or more parent commits, in which case we call it a "merge", due to the
 fact that such a commit brings together ("merges") two or more
 previous states represented by other commits.
 
 In other words, while a "tree" represents a particular directory state
 of a working directory, a "commit" represents that state in "time",
 and explains how we got there.
 
 You create a commit object by giving it the tree that describes the
 state at the time of the commit, and a list of parents:
 
 	git-commit-tree <tree> -p <parent> [-p <parent2> ..]
 
 and then giving the reason for the commit on stdin (either through
 redirection from a pipe or file, or by just typing it at the tty).
 
 git-commit-tree will return the name of the object that represents
 that commit, and you should save it away for later use. Normally,
 you'd commit a new `HEAD` state, and while git doesn't care where you
 save the note about that state, in practice we tend to just write the
 result to the file pointed at by `.git/HEAD`, so that we can always see
 what the last committed state was.
 
 Here is an ASCII art by Jon Loeliger that illustrates how
 various pieces fit together.
 
 ------------
 
                      commit-tree
                       commit obj
                        +----+
                        |    |
                        |    |
                        V    V
                     +-----------+
                     | Object DB |
                     |  Backing  |
                     |   Store   |
                     +-----------+
                        ^
            write-tree  |     |
              tree obj  |     |
                        |     |  read-tree
                        |     |  tree obj
                              V
                     +-----------+
                     |   Index   |
                     |  "cache"  |
                     +-----------+
          update-index  ^
              blob obj  |     |
                        |     |
     checkout-index -u  |     |  checkout-index
              stat      |     |  blob obj
                              V
                     +-----------+
                     |  Working  |
                     | Directory |
                     +-----------+
 
 ------------
 
 
 6) Examining the data
 ~~~~~~~~~~~~~~~~~~~~~
 
 You can examine the data represented in the object database and the
 index with various helper tools. For every object, you can use
 gitlink:git-cat-file[1] to examine details about the
 object:
 
 		git-cat-file -t <objectname>
 
 shows the type of the object, and once you have the type (which is
 usually implicit in where you find the object), you can use
 
 		git-cat-file blob|tree|commit|tag <objectname>
 
 to show its contents. NOTE! Trees have binary content, and as a result
 there is a special helper for showing that content, called
 `git-ls-tree`, which turns the binary content into a more easily
 readable form.
 
 It's especially instructive to look at "commit" objects, since those
 tend to be small and fairly self-explanatory. In particular, if you
 follow the convention of having the top commit name in `.git/HEAD`,
 you can do
 
 		git-cat-file commit HEAD
 
 to see what the top commit was.
 
 7) Merging multiple trees
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Git helps you do a three-way merge, which you can expand to n-way by
 repeating the merge procedure arbitrary times until you finally
 "commit" the state.  The normal situation is that you'd only do one
 three-way merge (two parents), and commit it, but if you like to, you
 can do multiple parents in one go.
 
 To do a three-way merge, you need the two sets of "commit" objects
 that you want to merge, use those to find the closest common parent (a
 third "commit" object), and then use those commit objects to find the
 state of the directory ("tree" object) at these points.
 
 To get the "base" for the merge, you first look up the common parent
 of two commits with
 
 		git-merge-base <commit1> <commit2>
 
 which will return you the commit they are both based on.  You should
 now look up the "tree" objects of those commits, which you can easily
 do with (for example)
 
 		git-cat-file commit <commitname> | head -1
 
 since the tree object information is always the first line in a commit
 object.
 
 Once you know the three trees you are going to merge (the one
 "original" tree, aka the common case, and the two "result" trees, aka
 the branches you want to merge), you do a "merge" read into the
 index. This will complain if it has to throw away your old index contents, so you should
 make sure that you've committed those - in fact you would normally
 always do a merge against your last commit (which should thus match
 what you have in your current index anyway).
 
 To do the merge, do
 
 		git-read-tree -m -u <origtree> <yourtree> <targettree>
 
 which will do all trivial merge operations for you directly in the
 index file, and you can just write the result out with
 `git-write-tree`.
 
 Historical note.  We did not have `-u` facility when this
 section was first written, so we used to warn that
 the merge is done in the index file, not in your
 -working directory, and your working directory will no longer match your
 -index.
 +working tree, and your working tree will not match your
 +index after this step.
 +This is no longer true.  The above command, thanks to `-u`
 +option, updates your working tree with the merge results for
 +paths that have been trivially merged.
 
 
 8) Merging multiple trees, continued
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Sadly, many merges aren't trivial. If there are files that have
 been added.moved or removed, or if both branches have modified the
 same file, you will be left with an index tree that contains "merge
 entries" in it. Such an index tree can 'NOT' be written out to a tree
 object, and you will have to resolve any such merge clashes using
 other tools before you can write out the result.
 
 You can examine such index state with `git-ls-files --unmerged`
 command.  An example:
 
 ------------------------------------------------
 $ git-read-tree -m $orig HEAD $target
 $ git-ls-files --unmerged
 100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1	hello.c
 100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2	hello.c
 100644 cc44c73eb783565da5831b4d820c962954019b69 3	hello.c
 ------------------------------------------------
 
 Each line of the `git-ls-files --unmerged` output begins with
 the blob mode bits, blob SHA1, 'stage number', and the
 filename.  The 'stage number' is git's way to say which tree it
 came from: stage 1 corresponds to `$orig` tree, stage 2 `HEAD`
 tree, and stage3 `$target` tree.
 
 Earlier we said that trivial merges are done inside
 `git-read-tree -m`.  For example, if the file did not change
 from `$orig` to `HEAD` nor `$target`, or if the file changed
 from `$orig` to `HEAD` and `$orig` to `$target` the same way,
 obviously the final outcome is what is in `HEAD`.  What the
 above example shows is that file `hello.c` was changed from
 `$orig` to `HEAD` and `$orig` to `$target` in a different way.
 You could resolve this by running your favorite 3-way merge
 program, e.g.  `diff3` or `merge`, on the blob objects from
 these three stages yourself, like this:
 
 ------------------------------------------------
 $ git-cat-file blob 263414f... >hello.c~1
 $ git-cat-file blob 06fa6a2... >hello.c~2
 $ git-cat-file blob cc44c73... >hello.c~3
 $ merge hello.c~2 hello.c~1 hello.c~3
 ------------------------------------------------
 
 This would leave the merge result in `hello.c~2` file, along
 with conflict markers if there are conflicts.  After verifying
 the merge result makes sense, you can tell git what the final
 merge result for this file is by:
 
 	mv -f hello.c~2 hello.c
 	git-update-index hello.c
 
 When a path is in unmerged state, running `git-update-index` for
 that path tells git to mark the path resolved.
 
 The above is the description of a git merge at the lowest level,
 to help you understand what conceptually happens under the hood.
 In practice, nobody, not even git itself, uses three `git-cat-file`
 for this.  There is `git-merge-index` program that extracts the
 -stages to temporary files and calls a `merge` script on it
 +stages to temporary files and calls a "merge" script on it:
 
 	git-merge-index git-merge-one-file hello.c
 
 and that is what higher level `git resolve` is implemented with.

 commit cd0a781c386b197e63a30104bead39420eada7ca
 Author: Junio C Hamano <[email protected]>
 Date:   Tue Nov 15 01:31:04 2005 -0800

    Documentation: do not blindly run 'cat' .git/HEAD, or echo into it.
    
    Many places in the documentation we still talked about reading
    what commit is recorded in .git/HEAD or writing the new head
    information into it, both assuming .git/HEAD is a symlink.  That
    is not necessarily so.
    
    Signed-off-by: Junio C Hamano <[email protected]>

 diff --git a/README b/README
 --- a/README
 +++ b/README
 @@ -17,570 +17,570 @@
 This is a stupid (but extremely fast) directory content manager.  It
 doesn't do a whole lot, but what it 'does' do is track directory
 contents efficiently. 
 
 There are two object abstractions: the "object database", and the
 "current directory cache" aka "index".
 
 The Object Database
 ~~~~~~~~~~~~~~~~~~~
 The object database is literally just a content-addressable collection
 of objects.  All objects are named by their content, which is
 approximated by the SHA1 hash of the object itself.  Objects may refer
 to other objects (by referencing their SHA1 hash), and so you can
 build up a hierarchy of objects.
 
 All objects have a statically determined "type" aka "tag", which is
 determined at object creation time, and which identifies the format of
 the object (i.e. how it is used, and how it can refer to other
 objects).  There are currently four different object types: "blob",
 "tree", "commit" and "tag".
 
 A "blob" object cannot refer to any other object, and is, like the tag
 implies, a pure storage object containing some user data.  It is used to
 actually store the file data, i.e. a blob object is associated with some
 particular version of some file. 
 
 A "tree" object is an object that ties one or more "blob" objects into a
 directory structure. In addition, a tree object can refer to other tree
 objects, thus creating a directory hierarchy. 
 
 A "commit" object ties such directory hierarchies together into
 a DAG of revisions - each "commit" is associated with exactly one tree
 (the directory hierarchy at the time of the commit). In addition, a
 "commit" refers to one or more "parent" commit objects that describe the
 history of how we arrived at that directory hierarchy.
 
 As a special case, a commit object with no parents is called the "root"
 object, and is the point of an initial project commit.  Each project
 must have at least one root, and while you can tie several different
 root objects together into one project by creating a commit object which
 has two or more separate roots as its ultimate parents, that's probably
 just going to confuse people.  So aim for the notion of "one root object
 per project", even if git itself does not enforce that. 
 
 A "tag" object symbolically identifies and can be used to sign other
 objects. It contains the identifier and type of another object, a
 symbolic name (of course!) and, optionally, a signature.
 
 Regardless of object type, all objects share the following
 characteristics: they are all deflated with zlib, and have a header
 that not only specifies their tag, but also provides size information
 about the data in the object.  It's worth noting that the SHA1 hash
 that is used to name the object is the hash of the original data
 plus this header, so `sha1sum` 'file' does not match the object name
 for 'file'.
 (Historical note: in the dawn of the age of git the hash
 was the sha1 of the 'compressed' object.)
 
 As a result, the general consistency of an object can always be tested
 independently of the contents or the type of the object: all objects can
 be validated by verifying that (a) their hashes match the content of the
 file and (b) the object successfully inflates to a stream of bytes that
 forms a sequence of <ascii tag without space> + <space> + <ascii decimal
 size> + <byte\0> + <binary object data>. 
 
 The structured objects can further have their structure and
 connectivity to other objects verified. This is generally done with
 the `git-fsck-objects` program, which generates a full dependency graph
 of all objects, and verifies their internal consistency (in addition
 to just verifying their superficial consistency through the hash).
 
 The object types in some more detail:
 
 Blob Object
 ~~~~~~~~~~~
 A "blob" object is nothing but a binary blob of data, and doesn't
 refer to anything else.  There is no signature or any other
 verification of the data, so while the object is consistent (it 'is'
 indexed by its sha1 hash, so the data itself is certainly correct), it
 has absolutely no other attributes.  No name associations, no
 permissions.  It is purely a blob of data (i.e. normally "file
 contents").
 
 In particular, since the blob is entirely defined by its data, if two
 files in a directory tree (or in multiple different versions of the
 repository) have the same contents, they will share the same blob
 object. The object is totally independent of its location in the
 directory tree, and renaming a file does not change the object that
 file is associated with in any way.
 
 A blob is typically created when gitlink:git-update-index[1]
 is run, and its data can be accessed by gitlink:git-cat-file[1].
 
 Tree Object
 ~~~~~~~~~~~
 The next hierarchical object type is the "tree" object.  A tree object
 is a list of mode/name/blob data, sorted by name.  Alternatively, the
 mode data may specify a directory mode, in which case instead of
 naming a blob, that name is associated with another TREE object.
 
 Like the "blob" object, a tree object is uniquely determined by the
 set contents, and so two separate but identical trees will always
 share the exact same object. This is true at all levels, i.e. it's
 true for a "leaf" tree (which does not refer to any other trees, only
 blobs) as well as for a whole subdirectory.
 
 For that reason a "tree" object is just a pure data abstraction: it
 has no history, no signatures, no verification of validity, except
 that since the contents are again protected by the hash itself, we can
 trust that the tree is immutable and its contents never change.
 
 So you can trust the contents of a tree to be valid, the same way you
 can trust the contents of a blob, but you don't know where those
 contents 'came' from.
 
 Side note on trees: since a "tree" object is a sorted list of
 "filename+content", you can create a diff between two trees without
 actually having to unpack two trees.  Just ignore all common parts,
 and your diff will look right.  In other words, you can effectively
 (and efficiently) tell the difference between any two random trees by
 O(n) where "n" is the size of the difference, rather than the size of
 the tree.
 
 Side note 2 on trees: since the name of a "blob" depends entirely and
 exclusively on its contents (i.e. there are no names or permissions
 involved), you can see trivial renames or permission changes by
 noticing that the blob stayed the same.  However, renames with data
 changes need a smarter "diff" implementation.
 
 A tree is created with gitlink:git-write-tree[1] and
 its data can be accessed by gitlink:git-ls-tree[1].
 Two trees can be compared with gitlink:git-diff-tree[1].
 
 Commit Object
 ~~~~~~~~~~~~~
 The "commit" object is an object that introduces the notion of
 history into the picture.  In contrast to the other objects, it
 doesn't just describe the physical state of a tree, it describes how
 we got there, and why.
 
 A "commit" is defined by the tree-object that it results in, the
 parent commits (zero, one or more) that led up to that point, and a
 comment on what happened.  Again, a commit is not trusted per se:
 the contents are well-defined and "safe" due to the cryptographically
 strong signatures at all levels, but there is no reason to believe
 that the tree is "good" or that the merge information makes sense.
 The parents do not have to actually have any relationship with the
 result, for example.
 
 Note on commits: unlike real SCM's, commits do not contain
 rename information or file mode change information.  All of that is
 implicit in the trees involved (the result tree, and the result trees
 of the parents), and describing that makes no sense in this idiotic
 file manager.
 
 A commit is created with gitlink:git-commit-tree[1] and
 its data can be accessed by gitlink:git-cat-file[1].
 
 Trust
 ~~~~~
 An aside on the notion of "trust". Trust is really outside the scope
 of "git", but it's worth noting a few things.  First off, since
 everything is hashed with SHA1, you 'can' trust that an object is
 intact and has not been messed with by external sources.  So the name
 of an object uniquely identifies a known state - just not a state that
 you may want to trust.
 
 Furthermore, since the SHA1 signature of a commit refers to the
 SHA1 signatures of the tree it is associated with and the signatures
 of the parent, a single named commit specifies uniquely a whole set
 of history, with full contents.  You can't later fake any step of the
 way once you have the name of a commit.
 
 So to introduce some real trust in the system, the only thing you need
 to do is to digitally sign just 'one' special note, which includes the
 name of a top-level commit.  Your digital signature shows others
 that you trust that commit, and the immutability of the history of
 commits tells others that they can trust the whole history.
 
 In other words, you can easily validate a whole archive by just
 sending out a single email that tells the people the name (SHA1 hash)
 of the top commit, and digitally sign that email using something
 like GPG/PGP.
 
 To assist in this, git also provides the tag object...
 
 Tag Object
 ~~~~~~~~~~
 Git provides the "tag" object to simplify creating, managing and
 exchanging symbolic and signed tokens.  The "tag" object at its
 simplest simply symbolically identifies another object by containing
 the sha1, type and symbolic name.
 
 However it can optionally contain additional signature information
 (which git doesn't care about as long as there's less than 8k of
 it). This can then be verified externally to git.
 
 Note that despite the tag features, "git" itself only handles content
 integrity; the trust framework (and signature provision and
 verification) has to come from outside.
 
 A tag is created with gitlink:git-mktag[1],
 its data can be accessed by gitlink:git-cat-file[1],
 and the signature can be verified by
 gitlink:git-verify-tag[1].
 
 
 The "index" aka "Current Directory Cache"
 -----------------------------------------
 The index is a simple binary file, which contains an efficient
 representation of a virtual directory content at some random time.  It
 does so by a simple array that associates a set of names, dates,
 permissions and content (aka "blob") objects together.  The cache is
 always kept ordered by name, and names are unique (with a few very
 specific rules) at any point in time, but the cache has no long-term
 meaning, and can be partially updated at any time.
 
 In particular, the index certainly does not need to be consistent with
 the current directory contents (in fact, most operations will depend on
 different ways to make the index 'not' be consistent with the directory
 hierarchy), but it has three very important attributes:
 
 '(a) it can re-generate the full state it caches (not just the
 directory structure: it contains pointers to the "blob" objects so
 that it can regenerate the data too)'
 
 As a special case, there is a clear and unambiguous one-way mapping
 from a current directory cache to a "tree object", which can be
 efficiently created from just the current directory cache without
 actually looking at any other data.  So a directory cache at any one
 time uniquely specifies one and only one "tree" object (but has
 additional data to make it easy to match up that tree object with what
 has happened in the directory)
 
 '(b) it has efficient methods for finding inconsistencies between that
 cached state ("tree object waiting to be instantiated") and the
 current state.'
 
 '(c) it can additionally efficiently represent information about merge
 conflicts between different tree objects, allowing each pathname to be
 associated with sufficient information about the trees involved that
 you can create a three-way merge between them.'
 
 Those are the three ONLY things that the directory cache does.  It's a
 cache, and the normal operation is to re-generate it completely from a
 known tree object, or update/compare it with a live tree that is being
 developed.  If you blow the directory cache away entirely, you generally
 haven't lost any information as long as you have the name of the tree
 that it described. 
 
 At the same time, the index is at the same time also the
 staging area for creating new trees, and creating a new tree always
 involves a controlled modification of the index file.  In particular,
 the index file can have the representation of an intermediate tree that
 has not yet been instantiated.  So the index can be thought of as a
 write-back cache, which can contain dirty information that has not yet
 been written back to the backing store.
 
 
 
 The Workflow
 ------------
 Generally, all "git" operations work on the index file. Some operations
 work *purely* on the index file (showing the current state of the
 index), but most operations move data to and from the index file. Either
 from the database or from the working directory. Thus there are four
 main combinations: 
 
 1) working directory -> index
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You update the index with information from the working directory with
 the gitlink:git-update-index[1] command.  You
 generally update the index information by just specifying the filename
 you want to update, like so:
 
 	git-update-index filename
 
 but to avoid common mistakes with filename globbing etc, the command
 will not normally add totally new entries or remove old entries,
 i.e. it will normally just update existing cache entries.
 
 To tell git that yes, you really do realize that certain files no
 longer exist in the archive, or that new files should be added, you
 should use the `--remove` and `--add` flags respectively.
 
 NOTE! A `--remove` flag does 'not' mean that subsequent filenames will
 necessarily be removed: if the files still exist in your directory
 structure, the index will be updated with their new status, not
 removed. The only thing `--remove` means is that update-cache will be
 considering a removed file to be a valid thing, and if the file really
 does not exist any more, it will update the index accordingly.
 
 As a special case, you can also do `git-update-index --refresh`, which
 will refresh the "stat" information of each index to match the current
 stat information. It will 'not' update the object status itself, and
 it will only update the fields that are used to quickly test whether
 an object still matches its old backing store object.
 
 2) index -> object database
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You write your current index file to a "tree" object with the program
 
 	git-write-tree
 
 that doesn't come with any options - it will just write out the
 current index into the set of tree objects that describe that state,
 and it will return the name of the resulting top-level tree. You can
 use that tree to re-generate the index at any time by going in the
 other direction:
 
 3) object database -> index
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You read a "tree" file from the object database, and use that to
 populate (and overwrite - don't do this if your index contains any
 unsaved state that you might want to restore later!) your current
 index.  Normal operation is just
 
 		git-read-tree <sha1 of tree>
 
 and your index file will now be equivalent to the tree that you saved
 earlier. However, that is only your 'index' file: your working
 directory contents have not been modified.
 
 4) index -> working directory
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You update your working directory from the index by "checking out"
 files. This is not a very common operation, since normally you'd just
 keep your files updated, and rather than write to your working
 directory, you'd tell the index files about the changes in your
 working directory (i.e. `git-update-index`).
 
 However, if you decide to jump to a new version, or check out somebody
 else's version, or just restore a previous tree, you'd populate your
 index file with read-tree, and then you need to check out the result
 with
 
 		git-checkout-index filename
 
 or, if you want to check out all of the index, use `-a`.
 
 NOTE! git-checkout-index normally refuses to overwrite old files, so
 if you have an old version of the tree already checked out, you will
 need to use the "-f" flag ('before' the "-a" flag or the filename) to
 'force' the checkout.
 
 
 Finally, there are a few odds and ends which are not purely moving
 from one representation to the other:
 
 5) Tying it all together
 ~~~~~~~~~~~~~~~~~~~~~~~~
 To commit a tree you have instantiated with "git-write-tree", you'd
 create a "commit" object that refers to that tree and the history
 behind it - most notably the "parent" commits that preceded it in
 history.
 
 Normally a "commit" has one parent: the previous state of the tree
 before a certain change was made. However, sometimes it can have two
 or more parent commits, in which case we call it a "merge", due to the
 fact that such a commit brings together ("merges") two or more
 previous states represented by other commits.
 
 In other words, while a "tree" represents a particular directory state
 of a working directory, a "commit" represents that state in "time",
 and explains how we got there.
 
 You create a commit object by giving it the tree that describes the
 state at the time of the commit, and a list of parents:
 
 	git-commit-tree <tree> -p <parent> [-p <parent2> ..]
 
 and then giving the reason for the commit on stdin (either through
 redirection from a pipe or file, or by just typing it at the tty).
 
 git-commit-tree will return the name of the object that represents
 that commit, and you should save it away for later use. Normally,
 you'd commit a new `HEAD` state, and while git doesn't care where you
 save the note about that state, in practice we tend to just write the
 -result to the file `.git/HEAD`, so that we can always see what the
 -last committed state was.
 +result to the file pointed at by `.git/HEAD`, so that we can always see
 +what the last committed state was.
 
 Here is an ASCII art by Jon Loeliger that illustrates how
 various pieces fit together.
 
 ------------
 
                      commit-tree
                       commit obj
                        +----+
                        |    |
                        |    |
                        V    V
                     +-----------+
                     | Object DB |
                     |  Backing  |
                     |   Store   |
                     +-----------+
                        ^
            write-tree  |     |
              tree obj  |     |
                        |     |  read-tree
                        |     |  tree obj
                              V
                     +-----------+
                     |   Index   |
                     |  "cache"  |
                     +-----------+
          update-index  ^
              blob obj  |     |
                        |     |
     checkout-index -u  |     |  checkout-index
              stat      |     |  blob obj
                              V
                     +-----------+
                     |  Working  |
                     | Directory |
                     +-----------+
 
 ------------
 
 
 6) Examining the data
 ~~~~~~~~~~~~~~~~~~~~~
 
 You can examine the data represented in the object database and the
 index with various helper tools. For every object, you can use
 gitlink:git-cat-file[1] to examine details about the
 object:
 
 		git-cat-file -t <objectname>
 
 shows the type of the object, and once you have the type (which is
 usually implicit in where you find the object), you can use
 
 		git-cat-file blob|tree|commit|tag <objectname>
 
 to show its contents. NOTE! Trees have binary content, and as a result
 there is a special helper for showing that content, called
 `git-ls-tree`, which turns the binary content into a more easily
 readable form.
 
 It's especially instructive to look at "commit" objects, since those
 tend to be small and fairly self-explanatory. In particular, if you
 follow the convention of having the top commit name in `.git/HEAD`,
 you can do
 
 -		git-cat-file commit $(cat .git/HEAD)
 +		git-cat-file commit HEAD
 
 to see what the top commit was.
 
 7) Merging multiple trees
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Git helps you do a three-way merge, which you can expand to n-way by
 repeating the merge procedure arbitrary times until you finally
 "commit" the state.  The normal situation is that you'd only do one
 three-way merge (two parents), and commit it, but if you like to, you
 can do multiple parents in one go.
 
 To do a three-way merge, you need the two sets of "commit" objects
 that you want to merge, use those to find the closest common parent (a
 third "commit" object), and then use those commit objects to find the
 state of the directory ("tree" object) at these points.
 
 To get the "base" for the merge, you first look up the common parent
 of two commits with
 
 		git-merge-base <commit1> <commit2>
 
 which will return you the commit they are both based on.  You should
 now look up the "tree" objects of those commits, which you can easily
 do with (for example)
 
 		git-cat-file commit <commitname> | head -1
 
 since the tree object information is always the first line in a commit
 object.
 
 Once you know the three trees you are going to merge (the one
 "original" tree, aka the common case, and the two "result" trees, aka
 the branches you want to merge), you do a "merge" read into the
 index. This will complain if it has to throw away your old index contents, so you should
 make sure that you've committed those - in fact you would normally
 always do a merge against your last commit (which should thus match
 what you have in your current index anyway).
 
 To do the merge, do
 
 		git-read-tree -m -u <origtree> <yourtree> <targettree>
 
 which will do all trivial merge operations for you directly in the
 index file, and you can just write the result out with
 `git-write-tree`.
 
 Historical note.  We did not have `-u` facility when this
 section was first written, so we used to warn that
 the merge is done in the index file, not in your
 working directory, and your working directory will no longer match your
 index.
 
 
 8) Merging multiple trees, continued
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Sadly, many merges aren't trivial. If there are files that have
 been added.moved or removed, or if both branches have modified the
 same file, you will be left with an index tree that contains "merge
 entries" in it. Such an index tree can 'NOT' be written out to a tree
 object, and you will have to resolve any such merge clashes using
 other tools before you can write out the result.
 
 You can examine such index state with `git-ls-files --unmerged`
 command.  An example:
 
 ------------------------------------------------
 $ git-read-tree -m $orig HEAD $target
 $ git-ls-files --unmerged
 100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1	hello.c
 100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2	hello.c
 100644 cc44c73eb783565da5831b4d820c962954019b69 3	hello.c
 ------------------------------------------------
 
 Each line of the `git-ls-files --unmerged` output begins with
 the blob mode bits, blob SHA1, 'stage number', and the
 filename.  The 'stage number' is git's way to say which tree it
 came from: stage 1 corresponds to `$orig` tree, stage 2 `HEAD`
 tree, and stage3 `$target` tree.
 
 Earlier we said that trivial merges are done inside
 `git-read-tree -m`.  For example, if the file did not change
 from `$orig` to `HEAD` nor `$target`, or if the file changed
 from `$orig` to `HEAD` and `$orig` to `$target` the same way,
 obviously the final outcome is what is in `HEAD`.  What the
 above example shows is that file `hello.c` was changed from
 `$orig` to `HEAD` and `$orig` to `$target` in a different way.
 You could resolve this by running your favorite 3-way merge
 program, e.g.  `diff3` or `merge`, on the blob objects from
 these three stages yourself, like this:
 
 ------------------------------------------------
 $ git-cat-file blob 263414f... >hello.c~1
 $ git-cat-file blob 06fa6a2... >hello.c~2
 $ git-cat-file blob cc44c73... >hello.c~3
 $ merge hello.c~2 hello.c~1 hello.c~3
 ------------------------------------------------
 
 This would leave the merge result in `hello.c~2` file, along
 with conflict markers if there are conflicts.  After verifying
 the merge result makes sense, you can tell git what the final
 merge result for this file is by:
 
 	mv -f hello.c~2 hello.c
 	git-update-index hello.c
 
 When a path is in unmerged state, running `git-update-index` for
 that path tells git to mark the path resolved.
 
 The above is the description of a git merge at the lowest level,
 to help you understand what conceptually happens under the hood.
 In practice, nobody, not even git itself, uses three `git-cat-file`
 for this.  There is `git-merge-index` program that extracts the
 stages to temporary files and calls a `merge` script on it
 
 	git-merge-index git-merge-one-file hello.c
 
 and that is what higher level `git resolve` is implemented with.

 commit 66158e331b385a81ac825c208c6160a0cdd2324c
 Author: Junio C Hamano <[email protected]>
 Date:   Thu Nov 3 13:52:44 2005 -0800

    Illustration: "Fundamental Git Index Operations"
    
    Jon Loeliger's ASCII art in the Discussion section.
    
    Signed-off-by: Junio C Hamano <[email protected]>

 diff --git a/README b/README
 --- a/README
 +++ b/README
 @@ -17,530 +17,570 @@
 This is a stupid (but extremely fast) directory content manager.  It
 doesn't do a whole lot, but what it 'does' do is track directory
 contents efficiently. 
 
 There are two object abstractions: the "object database", and the
 "current directory cache" aka "index".
 
 The Object Database
 ~~~~~~~~~~~~~~~~~~~
 The object database is literally just a content-addressable collection
 of objects.  All objects are named by their content, which is
 approximated by the SHA1 hash of the object itself.  Objects may refer
 to other objects (by referencing their SHA1 hash), and so you can
 build up a hierarchy of objects.
 
 All objects have a statically determined "type" aka "tag", which is
 determined at object creation time, and which identifies the format of
 the object (i.e. how it is used, and how it can refer to other
 objects).  There are currently four different object types: "blob",
 "tree", "commit" and "tag".
 
 A "blob" object cannot refer to any other object, and is, like the tag
 implies, a pure storage object containing some user data.  It is used to
 actually store the file data, i.e. a blob object is associated with some
 particular version of some file. 
 
 A "tree" object is an object that ties one or more "blob" objects into a
 directory structure. In addition, a tree object can refer to other tree
 objects, thus creating a directory hierarchy. 
 
 A "commit" object ties such directory hierarchies together into
 a DAG of revisions - each "commit" is associated with exactly one tree
 (the directory hierarchy at the time of the commit). In addition, a
 "commit" refers to one or more "parent" commit objects that describe the
 history of how we arrived at that directory hierarchy.
 
 As a special case, a commit object with no parents is called the "root"
 object, and is the point of an initial project commit.  Each project
 must have at least one root, and while you can tie several different
 root objects together into one project by creating a commit object which
 has two or more separate roots as its ultimate parents, that's probably
 just going to confuse people.  So aim for the notion of "one root object
 per project", even if git itself does not enforce that. 
 
 A "tag" object symbolically identifies and can be used to sign other
 objects. It contains the identifier and type of another object, a
 symbolic name (of course!) and, optionally, a signature.
 
 Regardless of object type, all objects share the following
 characteristics: they are all deflated with zlib, and have a header
 that not only specifies their tag, but also provides size information
 about the data in the object.  It's worth noting that the SHA1 hash
 that is used to name the object is the hash of the original data
 plus this header, so `sha1sum` 'file' does not match the object name
 for 'file'.
 (Historical note: in the dawn of the age of git the hash
 was the sha1 of the 'compressed' object.)
 
 As a result, the general consistency of an object can always be tested
 independently of the contents or the type of the object: all objects can
 be validated by verifying that (a) their hashes match the content of the
 file and (b) the object successfully inflates to a stream of bytes that
 forms a sequence of <ascii tag without space> + <space> + <ascii decimal
 size> + <byte\0> + <binary object data>. 
 
 The structured objects can further have their structure and
 connectivity to other objects verified. This is generally done with
 the `git-fsck-objects` program, which generates a full dependency graph
 of all objects, and verifies their internal consistency (in addition
 to just verifying their superficial consistency through the hash).
 
 The object types in some more detail:
 
 Blob Object
 ~~~~~~~~~~~
 A "blob" object is nothing but a binary blob of data, and doesn't
 refer to anything else.  There is no signature or any other
 verification of the data, so while the object is consistent (it 'is'
 indexed by its sha1 hash, so the data itself is certainly correct), it
 has absolutely no other attributes.  No name associations, no
 permissions.  It is purely a blob of data (i.e. normally "file
 contents").
 
 In particular, since the blob is entirely defined by its data, if two
 files in a directory tree (or in multiple different versions of the
 repository) have the same contents, they will share the same blob
 object. The object is totally independent of its location in the
 directory tree, and renaming a file does not change the object that
 file is associated with in any way.
 
 A blob is typically created when gitlink:git-update-index[1]
 is run, and its data can be accessed by gitlink:git-cat-file[1].
 
 Tree Object
 ~~~~~~~~~~~
 The next hierarchical object type is the "tree" object.  A tree object
 is a list of mode/name/blob data, sorted by name.  Alternatively, the
 mode data may specify a directory mode, in which case instead of
 naming a blob, that name is associated with another TREE object.
 
 Like the "blob" object, a tree object is uniquely determined by the
 set contents, and so two separate but identical trees will always
 share the exact same object. This is true at all levels, i.e. it's
 true for a "leaf" tree (which does not refer to any other trees, only
 blobs) as well as for a whole subdirectory.
 
 For that reason a "tree" object is just a pure data abstraction: it
 has no history, no signatures, no verification of validity, except
 that since the contents are again protected by the hash itself, we can
 trust that the tree is immutable and its contents never change.
 
 So you can trust the contents of a tree to be valid, the same way you
 can trust the contents of a blob, but you don't know where those
 contents 'came' from.
 
 Side note on trees: since a "tree" object is a sorted list of
 "filename+content", you can create a diff between two trees without
 actually having to unpack two trees.  Just ignore all common parts,
 and your diff will look right.  In other words, you can effectively
 (and efficiently) tell the difference between any two random trees by
 O(n) where "n" is the size of the difference, rather than the size of
 the tree.
 
 Side note 2 on trees: since the name of a "blob" depends entirely and
 exclusively on its contents (i.e. there are no names or permissions
 involved), you can see trivial renames or permission changes by
 noticing that the blob stayed the same.  However, renames with data
 changes need a smarter "diff" implementation.
 
 A tree is created with gitlink:git-write-tree[1] and
 its data can be accessed by gitlink:git-ls-tree[1].
 Two trees can be compared with gitlink:git-diff-tree[1].
 
 Commit Object
 ~~~~~~~~~~~~~
 The "commit" object is an object that introduces the notion of
 history into the picture.  In contrast to the other objects, it
 doesn't just describe the physical state of a tree, it describes how
 we got there, and why.
 
 A "commit" is defined by the tree-object that it results in, the
 parent commits (zero, one or more) that led up to that point, and a
 comment on what happened.  Again, a commit is not trusted per se:
 the contents are well-defined and "safe" due to the cryptographically
 strong signatures at all levels, but there is no reason to believe
 that the tree is "good" or that the merge information makes sense.
 The parents do not have to actually have any relationship with the
 result, for example.
 
 Note on commits: unlike real SCM's, commits do not contain
 rename information or file mode change information.  All of that is
 implicit in the trees involved (the result tree, and the result trees
 of the parents), and describing that makes no sense in this idiotic
 file manager.
 
 A commit is created with gitlink:git-commit-tree[1] and
 its data can be accessed by gitlink:git-cat-file[1].
 
 Trust
 ~~~~~
 An aside on the notion of "trust". Trust is really outside the scope
 of "git", but it's worth noting a few things.  First off, since
 everything is hashed with SHA1, you 'can' trust that an object is
 intact and has not been messed with by external sources.  So the name
 of an object uniquely identifies a known state - just not a state that
 you may want to trust.
 
 Furthermore, since the SHA1 signature of a commit refers to the
 SHA1 signatures of the tree it is associated with and the signatures
 of the parent, a single named commit specifies uniquely a whole set
 of history, with full contents.  You can't later fake any step of the
 way once you have the name of a commit.
 
 So to introduce some real trust in the system, the only thing you need
 to do is to digitally sign just 'one' special note, which includes the
 name of a top-level commit.  Your digital signature shows others
 that you trust that commit, and the immutability of the history of
 commits tells others that they can trust the whole history.
 
 In other words, you can easily validate a whole archive by just
 sending out a single email that tells the people the name (SHA1 hash)
 of the top commit, and digitally sign that email using something
 like GPG/PGP.
 
 To assist in this, git also provides the tag object...
 
 Tag Object
 ~~~~~~~~~~
 Git provides the "tag" object to simplify creating, managing and
 exchanging symbolic and signed tokens.  The "tag" object at its
 simplest simply symbolically identifies another object by containing
 the sha1, type and symbolic name.
 
 However it can optionally contain additional signature information
 (which git doesn't care about as long as there's less than 8k of
 it). This can then be verified externally to git.
 
 Note that despite the tag features, "git" itself only handles content
 integrity; the trust framework (and signature provision and
 verification) has to come from outside.
 
 A tag is created with gitlink:git-mktag[1],
 its data can be accessed by gitlink:git-cat-file[1],
 and the signature can be verified by
 gitlink:git-verify-tag[1].
 
 
 The "index" aka "Current Directory Cache"
 -----------------------------------------
 The index is a simple binary file, which contains an efficient
 representation of a virtual directory content at some random time.  It
 does so by a simple array that associates a set of names, dates,
 permissions and content (aka "blob") objects together.  The cache is
 always kept ordered by name, and names are unique (with a few very
 specific rules) at any point in time, but the cache has no long-term
 meaning, and can be partially updated at any time.
 
 In particular, the index certainly does not need to be consistent with
 the current directory contents (in fact, most operations will depend on
 different ways to make the index 'not' be consistent with the directory
 hierarchy), but it has three very important attributes:
 
 '(a) it can re-generate the full state it caches (not just the
 directory structure: it contains pointers to the "blob" objects so
 that it can regenerate the data too)'
 
 As a special case, there is a clear and unambiguous one-way mapping
 from a current directory cache to a "tree object", which can be
 efficiently created from just the current directory cache without
 actually looking at any other data.  So a directory cache at any one
 time uniquely specifies one and only one "tree" object (but has
 additional data to make it easy to match up that tree object with what
 has happened in the directory)
 
 '(b) it has efficient methods for finding inconsistencies between that
 cached state ("tree object waiting to be instantiated") and the
 current state.'
 
 '(c) it can additionally efficiently represent information about merge
 conflicts between different tree objects, allowing each pathname to be
 associated with sufficient information about the trees involved that
 you can create a three-way merge between them.'
 
 Those are the three ONLY things that the directory cache does.  It's a
 cache, and the normal operation is to re-generate it completely from a
 known tree object, or update/compare it with a live tree that is being
 developed.  If you blow the directory cache away entirely, you generally
 haven't lost any information as long as you have the name of the tree
 that it described. 
 
 At the same time, the index is at the same time also the
 staging area for creating new trees, and creating a new tree always
 involves a controlled modification of the index file.  In particular,
 the index file can have the representation of an intermediate tree that
 has not yet been instantiated.  So the index can be thought of as a
 write-back cache, which can contain dirty information that has not yet
 been written back to the backing store.
 
 
 
 The Workflow
 ------------
 Generally, all "git" operations work on the index file. Some operations
 work *purely* on the index file (showing the current state of the
 index), but most operations move data to and from the index file. Either
 from the database or from the working directory. Thus there are four
 main combinations: 
 
 1) working directory -> index
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You update the index with information from the working directory with
 the gitlink:git-update-index[1] command.  You
 generally update the index information by just specifying the filename
 you want to update, like so:
 
 	git-update-index filename
 
 but to avoid common mistakes with filename globbing etc, the command
 will not normally add totally new entries or remove old entries,
 i.e. it will normally just update existing cache entries.
 
 To tell git that yes, you really do realize that certain files no
 longer exist in the archive, or that new files should be added, you
 should use the `--remove` and `--add` flags respectively.
 
 NOTE! A `--remove` flag does 'not' mean that subsequent filenames will
 necessarily be removed: if the files still exist in your directory
 structure, the index will be updated with their new status, not
 removed. The only thing `--remove` means is that update-cache will be
 considering a removed file to be a valid thing, and if the file really
 does not exist any more, it will update the index accordingly.
 
 As a special case, you can also do `git-update-index --refresh`, which
 will refresh the "stat" information of each index to match the current
 stat information. It will 'not' update the object status itself, and
 it will only update the fields that are used to quickly test whether
 an object still matches its old backing store object.
 
 2) index -> object database
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You write your current index file to a "tree" object with the program
 
 	git-write-tree
 
 that doesn't come with any options - it will just write out the
 current index into the set of tree objects that describe that state,
 and it will return the name of the resulting top-level tree. You can
 use that tree to re-generate the index at any time by going in the
 other direction:
 
 3) object database -> index
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You read a "tree" file from the object database, and use that to
 populate (and overwrite - don't do this if your index contains any
 unsaved state that you might want to restore later!) your current
 index.  Normal operation is just
 
 		git-read-tree <sha1 of tree>
 
 and your index file will now be equivalent to the tree that you saved
 earlier. However, that is only your 'index' file: your working
 directory contents have not been modified.
 
 4) index -> working directory
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You update your working directory from the index by "checking out"
 files. This is not a very common operation, since normally you'd just
 keep your files updated, and rather than write to your working
 directory, you'd tell the index files about the changes in your
 working directory (i.e. `git-update-index`).
 
 However, if you decide to jump to a new version, or check out somebody
 else's version, or just restore a previous tree, you'd populate your
 index file with read-tree, and then you need to check out the result
 with
 
 		git-checkout-index filename
 
 or, if you want to check out all of the index, use `-a`.
 
 NOTE! git-checkout-index normally refuses to overwrite old files, so
 if you have an old version of the tree already checked out, you will
 need to use the "-f" flag ('before' the "-a" flag or the filename) to
 'force' the checkout.
 
 
 Finally, there are a few odds and ends which are not purely moving
 from one representation to the other:
 
 5) Tying it all together
 ~~~~~~~~~~~~~~~~~~~~~~~~
 To commit a tree you have instantiated with "git-write-tree", you'd
 create a "commit" object that refers to that tree and the history
 behind it - most notably the "parent" commits that preceded it in
 history.
 
 Normally a "commit" has one parent: the previous state of the tree
 before a certain change was made. However, sometimes it can have two
 or more parent commits, in which case we call it a "merge", due to the
 fact that such a commit brings together ("merges") two or more
 previous states represented by other commits.
 
 In other words, while a "tree" represents a particular directory state
 of a working directory, a "commit" represents that state in "time",
 and explains how we got there.
 
 You create a commit object by giving it the tree that describes the
 state at the time of the commit, and a list of parents:
 
 	git-commit-tree <tree> -p <parent> [-p <parent2> ..]
 
 and then giving the reason for the commit on stdin (either through
 redirection from a pipe or file, or by just typing it at the tty).
 
 git-commit-tree will return the name of the object that represents
 that commit, and you should save it away for later use. Normally,
 you'd commit a new `HEAD` state, and while git doesn't care where you
 save the note about that state, in practice we tend to just write the
 result to the file `.git/HEAD`, so that we can always see what the
 last committed state was.
 
 +Here is an ASCII art by Jon Loeliger that illustrates how
 +various pieces fit together.
 +
 +------------
 +
 +                     commit-tree
 +                      commit obj
 +                       +----+
 +                       |    |
 +                       |    |
 +                       V    V
 +                    +-----------+
 +                    | Object DB |
 +                    |  Backing  |
 +                    |   Store   |
 +                    +-----------+
 +                       ^
 +           write-tree  |     |
 +             tree obj  |     |
 +                       |     |  read-tree
 +                       |     |  tree obj
 +                             V
 +                    +-----------+
 +                    |   Index   |
 +                    |  "cache"  |
 +                    +-----------+
 +         update-index  ^
 +             blob obj  |     |
 +                       |     |
 +    checkout-index -u  |     |  checkout-index
 +             stat      |     |  blob obj
 +                             V
 +                    +-----------+
 +                    |  Working  |
 +                    | Directory |
 +                    +-----------+
 +
 +------------
 +
 +
 6) Examining the data
 ~~~~~~~~~~~~~~~~~~~~~
 
 You can examine the data represented in the object database and the
 index with various helper tools. For every object, you can use
 gitlink:git-cat-file[1] to examine details about the
 object:
 
 		git-cat-file -t <objectname>
 
 shows the type of the object, and once you have the type (which is
 usually implicit in where you find the object), you can use
 
 		git-cat-file blob|tree|commit|tag <objectname>
 
 to show its contents. NOTE! Trees have binary content, and as a result
 there is a special helper for showing that content, called
 `git-ls-tree`, which turns the binary content into a more easily
 readable form.
 
 It's especially instructive to look at "commit" objects, since those
 tend to be small and fairly self-explanatory. In particular, if you
 follow the convention of having the top commit name in `.git/HEAD`,
 you can do
 
 		git-cat-file commit $(cat .git/HEAD)
 
 to see what the top commit was.
 
 7) Merging multiple trees
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Git helps you do a three-way merge, which you can expand to n-way by
 repeating the merge procedure arbitrary times until you finally
 "commit" the state.  The normal situation is that you'd only do one
 three-way merge (two parents), and commit it, but if you like to, you
 can do multiple parents in one go.
 
 To do a three-way merge, you need the two sets of "commit" objects
 that you want to merge, use those to find the closest common parent (a
 third "commit" object), and then use those commit objects to find the
 state of the directory ("tree" object) at these points.
 
 To get the "base" for the merge, you first look up the common parent
 of two commits with
 
 		git-merge-base <commit1> <commit2>
 
 which will return you the commit they are both based on.  You should
 now look up the "tree" objects of those commits, which you can easily
 do with (for example)
 
 		git-cat-file commit <commitname> | head -1
 
 since the tree object information is always the first line in a commit
 object.
 
 Once you know the three trees you are going to merge (the one
 "original" tree, aka the common case, and the two "result" trees, aka
 the branches you want to merge), you do a "merge" read into the
 index. This will complain if it has to throw away your old index contents, so you should
 make sure that you've committed those - in fact you would normally
 always do a merge against your last commit (which should thus match
 what you have in your current index anyway).
 
 To do the merge, do
 
 		git-read-tree -m -u <origtree> <yourtree> <targettree>
 
 which will do all trivial merge operations for you directly in the
 index file, and you can just write the result out with
 `git-write-tree`.
 
 Historical note.  We did not have `-u` facility when this
 section was first written, so we used to warn that
 the merge is done in the index file, not in your
 working directory, and your working directory will no longer match your
 index.
 
 
 8) Merging multiple trees, continued
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Sadly, many merges aren't trivial. If there are files that have
 been added.moved or removed, or if both branches have modified the
 same file, you will be left with an index tree that contains "merge
 entries" in it. Such an index tree can 'NOT' be written out to a tree
 object, and you will have to resolve any such merge clashes using
 other tools before you can write out the result.
 
 You can examine such index state with `git-ls-files --unmerged`
 command.  An example:
 
 ------------------------------------------------
 $ git-read-tree -m $orig HEAD $target
 $ git-ls-files --unmerged
 100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1	hello.c
 100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2	hello.c
 100644 cc44c73eb783565da5831b4d820c962954019b69 3	hello.c
 ------------------------------------------------
 
 Each line of the `git-ls-files --unmerged` output begins with
 the blob mode bits, blob SHA1, 'stage number', and the
 filename.  The 'stage number' is git's way to say which tree it
 came from: stage 1 corresponds to `$orig` tree, stage 2 `HEAD`
 tree, and stage3 `$target` tree.
 
 Earlier we said that trivial merges are done inside
 `git-read-tree -m`.  For example, if the file did not change
 from `$orig` to `HEAD` nor `$target`, or if the file changed
 from `$orig` to `HEAD` and `$orig` to `$target` the same way,
 obviously the final outcome is what is in `HEAD`.  What the
 above example shows is that file `hello.c` was changed from
 `$orig` to `HEAD` and `$orig` to `$target` in a different way.
 You could resolve this by running your favorite 3-way merge
 program, e.g.  `diff3` or `merge`, on the blob objects from
 these three stages yourself, like this:
 
 ------------------------------------------------
 $ git-cat-file blob 263414f... >hello.c~1
 $ git-cat-file blob 06fa6a2... >hello.c~2
 $ git-cat-file blob cc44c73... >hello.c~3
 $ merge hello.c~2 hello.c~1 hello.c~3
 ------------------------------------------------
 
 This would leave the merge result in `hello.c~2` file, along
 with conflict markers if there are conflicts.  After verifying
 the merge result makes sense, you can tell git what the final
 merge result for this file is by:
 
 	mv -f hello.c~2 hello.c
 	git-update-index hello.c
 
 When a path is in unmerged state, running `git-update-index` for
 that path tells git to mark the path resolved.
 
 The above is the description of a git merge at the lowest level,
 to help you understand what conceptually happens under the hood.
 In practice, nobody, not even git itself, uses three `git-cat-file`
 for this.  There is `git-merge-index` program that extracts the
 stages to temporary files and calls a `merge` script on it
 
 	git-merge-index git-merge-one-file hello.c
 
 and that is what higher level `git resolve` is implemented with.

 commit a7154e916c6fab01dfb67629dae8c4430daff669
 Author: Sergey Vlasov <[email protected]>
 Date:   Mon Sep 19 14:10:51 2005 +0400

    [PATCH] Documentation: Update all files to use the new gitlink: macro
    
    The replacement was performed automatically by these commands:
    
            perl -pi -e 's/link:(git.+)\.html\[\1\]/gitlink:$1\[1\]/g' \
                    README Documentation/*.txt
            perl -pi -e 's/link:git\.html\[git\]/gitlink:git\[7\]/g' \
                    README Documentation/*.txt
    
    Signed-off-by: Sergey Vlasov <[email protected]>
    Signed-off-by: Junio C Hamano <[email protected]>

 diff --git a/README b/README
 --- a/README
 +++ b/README
 @@ -17,530 +17,530 @@
 This is a stupid (but extremely fast) directory content manager.  It
 doesn't do a whole lot, but what it 'does' do is track directory
 contents efficiently. 
 
 There are two object abstractions: the "object database", and the
 "current directory cache" aka "index".
 
 The Object Database
 ~~~~~~~~~~~~~~~~~~~
 The object database is literally just a content-addressable collection
 of objects.  All objects are named by their content, which is
 approximated by the SHA1 hash of the object itself.  Objects may refer
 to other objects (by referencing their SHA1 hash), and so you can
 build up a hierarchy of objects.
 
 All objects have a statically determined "type" aka "tag", which is
 determined at object creation time, and which identifies the format of
 the object (i.e. how it is used, and how it can refer to other
 objects).  There are currently four different object types: "blob",
 "tree", "commit" and "tag".
 
 A "blob" object cannot refer to any other object, and is, like the tag
 implies, a pure storage object containing some user data.  It is used to
 actually store the file data, i.e. a blob object is associated with some
 particular version of some file. 
 
 A "tree" object is an object that ties one or more "blob" objects into a
 directory structure. In addition, a tree object can refer to other tree
 objects, thus creating a directory hierarchy. 
 
 A "commit" object ties such directory hierarchies together into
 a DAG of revisions - each "commit" is associated with exactly one tree
 (the directory hierarchy at the time of the commit). In addition, a
 "commit" refers to one or more "parent" commit objects that describe the
 history of how we arrived at that directory hierarchy.
 
 As a special case, a commit object with no parents is called the "root"
 object, and is the point of an initial project commit.  Each project
 must have at least one root, and while you can tie several different
 root objects together into one project by creating a commit object which
 has two or more separate roots as its ultimate parents, that's probably
 just going to confuse people.  So aim for the notion of "one root object
 per project", even if git itself does not enforce that. 
 
 A "tag" object symbolically identifies and can be used to sign other
 objects. It contains the identifier and type of another object, a
 symbolic name (of course!) and, optionally, a signature.
 
 Regardless of object type, all objects share the following
 characteristics: they are all deflated with zlib, and have a header
 that not only specifies their tag, but also provides size information
 about the data in the object.  It's worth noting that the SHA1 hash
 that is used to name the object is the hash of the original data
 plus this header, so `sha1sum` 'file' does not match the object name
 for 'file'.
 (Historical note: in the dawn of the age of git the hash
 was the sha1 of the 'compressed' object.)
 
 As a result, the general consistency of an object can always be tested
 independently of the contents or the type of the object: all objects can
 be validated by verifying that (a) their hashes match the content of the
 file and (b) the object successfully inflates to a stream of bytes that
 forms a sequence of <ascii tag without space> + <space> + <ascii decimal
 size> + <byte\0> + <binary object data>. 
 
 The structured objects can further have their structure and
 connectivity to other objects verified. This is generally done with
 the `git-fsck-objects` program, which generates a full dependency graph
 of all objects, and verifies their internal consistency (in addition
 to just verifying their superficial consistency through the hash).
 
 The object types in some more detail:
 
 Blob Object
 ~~~~~~~~~~~
 A "blob" object is nothing but a binary blob of data, and doesn't
 refer to anything else.  There is no signature or any other
 verification of the data, so while the object is consistent (it 'is'
 indexed by its sha1 hash, so the data itself is certainly correct), it
 has absolutely no other attributes.  No name associations, no
 permissions.  It is purely a blob of data (i.e. normally "file
 contents").
 
 In particular, since the blob is entirely defined by its data, if two
 files in a directory tree (or in multiple different versions of the
 repository) have the same contents, they will share the same blob
 object. The object is totally independent of its location in the
 directory tree, and renaming a file does not change the object that
 file is associated with in any way.
 
 -A blob is typically created when link:git-update-index.html[git-update-index]
 -is run, and its data can be accessed by link:git-cat-file.html[git-cat-file].
 +A blob is typically created when gitlink:git-update-index[1]
 +is run, and its data can be accessed by gitlink:git-cat-file[1].
 
 Tree Object
 ~~~~~~~~~~~
 The next hierarchical object type is the "tree" object.  A tree object
 is a list of mode/name/blob data, sorted by name.  Alternatively, the
 mode data may specify a directory mode, in which case instead of
 naming a blob, that name is associated with another TREE object.
 
 Like the "blob" object, a tree object is uniquely determined by the
 set contents, and so two separate but identical trees will always
 share the exact same object. This is true at all levels, i.e. it's
 true for a "leaf" tree (which does not refer to any other trees, only
 blobs) as well as for a whole subdirectory.
 
 For that reason a "tree" object is just a pure data abstraction: it
 has no history, no signatures, no verification of validity, except
 that since the contents are again protected by the hash itself, we can
 trust that the tree is immutable and its contents never change.
 
 So you can trust the contents of a tree to be valid, the same way you
 can trust the contents of a blob, but you don't know where those
 contents 'came' from.
 
 Side note on trees: since a "tree" object is a sorted list of
 "filename+content", you can create a diff between two trees without
 actually having to unpack two trees.  Just ignore all common parts,
 and your diff will look right.  In other words, you can effectively
 (and efficiently) tell the difference between any two random trees by
 O(n) where "n" is the size of the difference, rather than the size of
 the tree.
 
 Side note 2 on trees: since the name of a "blob" depends entirely and
 exclusively on its contents (i.e. there are no names or permissions
 involved), you can see trivial renames or permission changes by
 noticing that the blob stayed the same.  However, renames with data
 changes need a smarter "diff" implementation.
 
 -A tree is created with link:git-write-tree.html[git-write-tree] and
 -its data can be accessed by link:git-ls-tree.html[git-ls-tree].
 -Two trees can be compared with link:git-diff-tree.html[git-diff-tree].
 +A tree is created with gitlink:git-write-tree[1] and
 +its data can be accessed by gitlink:git-ls-tree[1].
 +Two trees can be compared with gitlink:git-diff-tree[1].
 
 Commit Object
 ~~~~~~~~~~~~~
 The "commit" object is an object that introduces the notion of
 history into the picture.  In contrast to the other objects, it
 doesn't just describe the physical state of a tree, it describes how
 we got there, and why.
 
 A "commit" is defined by the tree-object that it results in, the
 parent commits (zero, one or more) that led up to that point, and a
 comment on what happened.  Again, a commit is not trusted per se:
 the contents are well-defined and "safe" due to the cryptographically
 strong signatures at all levels, but there is no reason to believe
 that the tree is "good" or that the merge information makes sense.
 The parents do not have to actually have any relationship with the
 result, for example.
 
 Note on commits: unlike real SCM's, commits do not contain
 rename information or file mode change information.  All of that is
 implicit in the trees involved (the result tree, and the result trees
 of the parents), and describing that makes no sense in this idiotic
 file manager.
 
 -A commit is created with link:git-commit-tree.html[git-commit-tree] and
 -its data can be accessed by link:git-cat-file.html[git-cat-file].
 +A commit is created with gitlink:git-commit-tree[1] and
 +its data can be accessed by gitlink:git-cat-file[1].
 
 Trust
 ~~~~~
 An aside on the notion of "trust". Trust is really outside the scope
 of "git", but it's worth noting a few things.  First off, since
 everything is hashed with SHA1, you 'can' trust that an object is
 intact and has not been messed with by external sources.  So the name
 of an object uniquely identifies a known state - just not a state that
 you may want to trust.
 
 Furthermore, since the SHA1 signature of a commit refers to the
 SHA1 signatures of the tree it is associated with and the signatures
 of the parent, a single named commit specifies uniquely a whole set
 of history, with full contents.  You can't later fake any step of the
 way once you have the name of a commit.
 
 So to introduce some real trust in the system, the only thing you need
 to do is to digitally sign just 'one' special note, which includes the
 name of a top-level commit.  Your digital signature shows others
 that you trust that commit, and the immutability of the history of
 commits tells others that they can trust the whole history.
 
 In other words, you can easily validate a whole archive by just
 sending out a single email that tells the people the name (SHA1 hash)
 of the top commit, and digitally sign that email using something
 like GPG/PGP.
 
 To assist in this, git also provides the tag object...
 
 Tag Object
 ~~~~~~~~~~
 Git provides the "tag" object to simplify creating, managing and
 exchanging symbolic and signed tokens.  The "tag" object at its
 simplest simply symbolically identifies another object by containing
 the sha1, type and symbolic name.
 
 However it can optionally contain additional signature information
 (which git doesn't care about as long as there's less than 8k of
 it). This can then be verified externally to git.
 
 Note that despite the tag features, "git" itself only handles content
 integrity; the trust framework (and signature provision and
 verification) has to come from outside.
 
 -A tag is created with link:git-mktag.html[git-mktag],
 -its data can be accessed by link:git-cat-file.html[git-cat-file],
 +A tag is created with gitlink:git-mktag[1],
 +its data can be accessed by gitlink:git-cat-file[1],
 and the signature can be verified by
 -link:git-verify-tag.html[git-verify-tag].
 +gitlink:git-verify-tag[1].
 
 
 The "index" aka "Current Directory Cache"
 -----------------------------------------
 The index is a simple binary file, which contains an efficient
 representation of a virtual directory content at some random time.  It
 does so by a simple array that associates a set of names, dates,
 permissions and content (aka "blob") objects together.  The cache is
 always kept ordered by name, and names are unique (with a few very
 specific rules) at any point in time, but the cache has no long-term
 meaning, and can be partially updated at any time.
 
 In particular, the index certainly does not need to be consistent with
 the current directory contents (in fact, most operations will depend on
 different ways to make the index 'not' be consistent with the directory
 hierarchy), but it has three very important attributes:
 
 '(a) it can re-generate the full state it caches (not just the
 directory structure: it contains pointers to the "blob" objects so
 that it can regenerate the data too)'
 
 As a special case, there is a clear and unambiguous one-way mapping
 from a current directory cache to a "tree object", which can be
 efficiently created from just the current directory cache without
 actually looking at any other data.  So a directory cache at any one
 time uniquely specifies one and only one "tree" object (but has
 additional data to make it easy to match up that tree object with what
 has happened in the directory)
 
 '(b) it has efficient methods for finding inconsistencies between that
 cached state ("tree object waiting to be instantiated") and the
 current state.'
 
 '(c) it can additionally efficiently represent information about merge
 conflicts between different tree objects, allowing each pathname to be
 associated with sufficient information about the trees involved that
 you can create a three-way merge between them.'
 
 Those are the three ONLY things that the directory cache does.  It's a
 cache, and the normal operation is to re-generate it completely from a
 known tree object, or update/compare it with a live tree that is being
 developed.  If you blow the directory cache away entirely, you generally
 haven't lost any information as long as you have the name of the tree
 that it described. 
 
 At the same time, the index is at the same time also the
 staging area for creating new trees, and creating a new tree always
 involves a controlled modification of the index file.  In particular,
 the index file can have the representation of an intermediate tree that
 has not yet been instantiated.  So the index can be thought of as a
 write-back cache, which can contain dirty information that has not yet
 been written back to the backing store.
 
 
 
 The Workflow
 ------------
 Generally, all "git" operations work on the index file. Some operations
 work *purely* on the index file (showing the current state of the
 index), but most operations move data to and from the index file. Either
 from the database or from the working directory. Thus there are four
 main combinations: 
 
 1) working directory -> index
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You update the index with information from the working directory with
 -the link:git-update-index.html[git-update-index] command.  You
 +the gitlink:git-update-index[1] command.  You
 generally update the index information by just specifying the filename
 you want to update, like so:
 
 	git-update-index filename
 
 but to avoid common mistakes with filename globbing etc, the command
 will not normally add totally new entries or remove old entries,
 i.e. it will normally just update existing cache entries.
 
 To tell git that yes, you really do realize that certain files no
 longer exist in the archive, or that new files should be added, you
 should use the `--remove` and `--add` flags respectively.
 
 NOTE! A `--remove` flag does 'not' mean that subsequent filenames will
 necessarily be removed: if the files still exist in your directory
 structure, the index will be updated with their new status, not
 removed. The only thing `--remove` means is that update-cache will be
 considering a removed file to be a valid thing, and if the file really
 does not exist any more, it will update the index accordingly.
 
 As a special case, you can also do `git-update-index --refresh`, which
 will refresh the "stat" information of each index to match the current
 stat information. It will 'not' update the object status itself, and
 it will only update the fields that are used to quickly test whether
 an object still matches its old backing store object.
 
 2) index -> object database
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You write your current index file to a "tree" object with the program
 
 	git-write-tree
 
 that doesn't come with any options - it will just write out the
 current index into the set of tree objects that describe that state,
 and it will return the name of the resulting top-level tree. You can
 use that tree to re-generate the index at any time by going in the
 other direction:
 
 3) object database -> index
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You read a "tree" file from the object database, and use that to
 populate (and overwrite - don't do this if your index contains any
 unsaved state that you might want to restore later!) your current
 index.  Normal operation is just
 
 		git-read-tree <sha1 of tree>
 
 and your index file will now be equivalent to the tree that you saved
 earlier. However, that is only your 'index' file: your working
 directory contents have not been modified.
 
 4) index -> working directory
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You update your working directory from the index by "checking out"
 files. This is not a very common operation, since normally you'd just
 keep your files updated, and rather than write to your working
 directory, you'd tell the index files about the changes in your
 working directory (i.e. `git-update-index`).
 
 However, if you decide to jump to a new version, or check out somebody
 else's version, or just restore a previous tree, you'd populate your
 index file with read-tree, and then you need to check out the result
 with
 
 		git-checkout-index filename
 
 or, if you want to check out all of the index, use `-a`.
 
 NOTE! git-checkout-index normally refuses to overwrite old files, so
 if you have an old version of the tree already checked out, you will
 need to use the "-f" flag ('before' the "-a" flag or the filename) to
 'force' the checkout.
 
 
 Finally, there are a few odds and ends which are not purely moving
 from one representation to the other:
 
 5) Tying it all together
 ~~~~~~~~~~~~~~~~~~~~~~~~
 To commit a tree you have instantiated with "git-write-tree", you'd
 create a "commit" object that refers to that tree and the history
 behind it - most notably the "parent" commits that preceded it in
 history.
 
 Normally a "commit" has one parent: the previous state of the tree
 before a certain change was made. However, sometimes it can have two
 or more parent commits, in which case we call it a "merge", due to the
 fact that such a commit brings together ("merges") two or more
 previous states represented by other commits.
 
 In other words, while a "tree" represents a particular directory state
 of a working directory, a "commit" represents that state in "time",
 and explains how we got there.
 
 You create a commit object by giving it the tree that describes the
 state at the time of the commit, and a list of parents:
 
 	git-commit-tree <tree> -p <parent> [-p <parent2> ..]
 
 and then giving the reason for the commit on stdin (either through
 redirection from a pipe or file, or by just typing it at the tty).
 
 git-commit-tree will return the name of the object that represents
 that commit, and you should save it away for later use. Normally,
 you'd commit a new `HEAD` state, and while git doesn't care where you
 save the note about that state, in practice we tend to just write the
 result to the file `.git/HEAD`, so that we can always see what the
 last committed state was.
 
 6) Examining the data
 ~~~~~~~~~~~~~~~~~~~~~
 
 You can examine the data represented in the object database and the
 index with various helper tools. For every object, you can use
 -link:git-cat-file.html[git-cat-file] to examine details about the
 +gitlink:git-cat-file[1] to examine details about the
 object:
 
 		git-cat-file -t <objectname>
 
 shows the type of the object, and once you have the type (which is
 usually implicit in where you find the object), you can use
 
 		git-cat-file blob|tree|commit|tag <objectname>
 
 to show its contents. NOTE! Trees have binary content, and as a result
 there is a special helper for showing that content, called
 `git-ls-tree`, which turns the binary content into a more easily
 readable form.
 
 It's especially instructive to look at "commit" objects, since those
 tend to be small and fairly self-explanatory. In particular, if you
 follow the convention of having the top commit name in `.git/HEAD`,
 you can do
 
 		git-cat-file commit $(cat .git/HEAD)
 
 to see what the top commit was.
 
 7) Merging multiple trees
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Git helps you do a three-way merge, which you can expand to n-way by
 repeating the merge procedure arbitrary times until you finally
 "commit" the state.  The normal situation is that you'd only do one
 three-way merge (two parents), and commit it, but if you like to, you
 can do multiple parents in one go.
 
 To do a three-way merge, you need the two sets of "commit" objects
 that you want to merge, use those to find the closest common parent (a
 third "commit" object), and then use those commit objects to find the
 state of the directory ("tree" object) at these points.
 
 To get the "base" for the merge, you first look up the common parent
 of two commits with
 
 		git-merge-base <commit1> <commit2>
 
 which will return you the commit they are both based on.  You should
 now look up the "tree" objects of those commits, which you can easily
 do with (for example)
 
 		git-cat-file commit <commitname> | head -1
 
 since the tree object information is always the first line in a commit
 object.
 
 Once you know the three trees you are going to merge (the one
 "original" tree, aka the common case, and the two "result" trees, aka
 the branches you want to merge), you do a "merge" read into the
 index. This will complain if it has to throw away your old index contents, so you should
 make sure that you've committed those - in fact you would normally
 always do a merge against your last commit (which should thus match
 what you have in your current index anyway).
 
 To do the merge, do
 
 		git-read-tree -m -u <origtree> <yourtree> <targettree>
 
 which will do all trivial merge operations for you directly in the
 index file, and you can just write the result out with
 `git-write-tree`.
 
 Historical note.  We did not have `-u` facility when this
 section was first written, so we used to warn that
 the merge is done in the index file, not in your
 working directory, and your working directory will no longer match your
 index.
 
 
 8) Merging multiple trees, continued
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Sadly, many merges aren't trivial. If there are files that have
 been added.moved or removed, or if both branches have modified the
 same file, you will be left with an index tree that contains "merge
 entries" in it. Such an index tree can 'NOT' be written out to a tree
 object, and you will have to resolve any such merge clashes using
 other tools before you can write out the result.
 
 You can examine such index state with `git-ls-files --unmerged`
 command.  An example:
 
 ------------------------------------------------
 $ git-read-tree -m $orig HEAD $target
 $ git-ls-files --unmerged
 100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1	hello.c
 100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2	hello.c
 100644 cc44c73eb783565da5831b4d820c962954019b69 3	hello.c
 ------------------------------------------------
 
 Each line of the `git-ls-files --unmerged` output begins with
 the blob mode bits, blob SHA1, 'stage number', and the
 filename.  The 'stage number' is git's way to say which tree it
 came from: stage 1 corresponds to `$orig` tree, stage 2 `HEAD`
 tree, and stage3 `$target` tree.
 
 Earlier we said that trivial merges are done inside
 `git-read-tree -m`.  For example, if the file did not change
 from `$orig` to `HEAD` nor `$target`, or if the file changed
 from `$orig` to `HEAD` and `$orig` to `$target` the same way,
 obviously the final outcome is what is in `HEAD`.  What the
 above example shows is that file `hello.c` was changed from
 `$orig` to `HEAD` and `$orig` to `$target` in a different way.
 You could resolve this by running your favorite 3-way merge
 program, e.g.  `diff3` or `merge`, on the blob objects from
 these three stages yourself, like this:
 
 ------------------------------------------------
 $ git-cat-file blob 263414f... >hello.c~1
 $ git-cat-file blob 06fa6a2... >hello.c~2
 $ git-cat-file blob cc44c73... >hello.c~3
 $ merge hello.c~2 hello.c~1 hello.c~3
 ------------------------------------------------
 
 This would leave the merge result in `hello.c~2` file, along
 with conflict markers if there are conflicts.  After verifying
 the merge result makes sense, you can tell git what the final
 merge result for this file is by:
 
 	mv -f hello.c~2 hello.c
 	git-update-index hello.c
 
 When a path is in unmerged state, running `git-update-index` for
 that path tells git to mark the path resolved.
 
 The above is the description of a git merge at the lowest level,
 to help you understand what conceptually happens under the hood.
 In practice, nobody, not even git itself, uses three `git-cat-file`
 for this.  There is `git-merge-index` program that extracts the
 stages to temporary files and calls a `merge` script on it
 
 	git-merge-index git-merge-one-file hello.c
 
 and that is what higher level `git resolve` is implemented with.

 commit 215a7ad1ef790467a4cd3f0dcffbd6e5f04c38f7
 Author: Junio C Hamano <[email protected]>
 Date:   Wed Sep 7 17:26:23 2005 -0700

    Big tool rename.
    
    As promised, this is the "big tool rename" patch.  The primary differences
    since 0.99.6 are:
    
      (1) git-*-script are no more.  The commands installed do not
          have any such suffix so users do not have to remember if
          something is implemented as a shell script or not.
    
      (2) Many command names with 'cache' in them are renamed with
          'index' if that is what they mean.
    
    There are backward compatibility symblic links so that you and
    Porcelains can keep using the old names, but the backward
    compatibility support  is expected to be removed in the near
    future.
    
    Signed-off-by: Junio C Hamano <[email protected]>

 diff --git a/README b/README
 --- a/README
 +++ b/README
 @@ -17,530 +17,530 @@
 This is a stupid (but extremely fast) directory content manager.  It
 doesn't do a whole lot, but what it 'does' do is track directory
 contents efficiently. 
 
 There are two object abstractions: the "object database", and the
 "current directory cache" aka "index".
 
 The Object Database
 ~~~~~~~~~~~~~~~~~~~
 The object database is literally just a content-addressable collection
 of objects.  All objects are named by their content, which is
 approximated by the SHA1 hash of the object itself.  Objects may refer
 to other objects (by referencing their SHA1 hash), and so you can
 build up a hierarchy of objects.
 
 All objects have a statically determined "type" aka "tag", which is
 determined at object creation time, and which identifies the format of
 the object (i.e. how it is used, and how it can refer to other
 objects).  There are currently four different object types: "blob",
 "tree", "commit" and "tag".
 
 A "blob" object cannot refer to any other object, and is, like the tag
 implies, a pure storage object containing some user data.  It is used to
 actually store the file data, i.e. a blob object is associated with some
 particular version of some file. 
 
 A "tree" object is an object that ties one or more "blob" objects into a
 directory structure. In addition, a tree object can refer to other tree
 objects, thus creating a directory hierarchy. 
 
 A "commit" object ties such directory hierarchies together into
 a DAG of revisions - each "commit" is associated with exactly one tree
 (the directory hierarchy at the time of the commit). In addition, a
 "commit" refers to one or more "parent" commit objects that describe the
 history of how we arrived at that directory hierarchy.
 
 As a special case, a commit object with no parents is called the "root"
 object, and is the point of an initial project commit.  Each project
 must have at least one root, and while you can tie several different
 root objects together into one project by creating a commit object which
 has two or more separate roots as its ultimate parents, that's probably
 just going to confuse people.  So aim for the notion of "one root object
 per project", even if git itself does not enforce that. 
 
 A "tag" object symbolically identifies and can be used to sign other
 objects. It contains the identifier and type of another object, a
 symbolic name (of course!) and, optionally, a signature.
 
 Regardless of object type, all objects share the following
 characteristics: they are all deflated with zlib, and have a header
 that not only specifies their tag, but also provides size information
 about the data in the object.  It's worth noting that the SHA1 hash
 that is used to name the object is the hash of the original data
 plus this header, so `sha1sum` 'file' does not match the object name
 for 'file'.
 (Historical note: in the dawn of the age of git the hash
 was the sha1 of the 'compressed' object.)
 
 As a result, the general consistency of an object can always be tested
 independently of the contents or the type of the object: all objects can
 be validated by verifying that (a) their hashes match the content of the
 file and (b) the object successfully inflates to a stream of bytes that
 forms a sequence of <ascii tag without space> + <space> + <ascii decimal
 size> + <byte\0> + <binary object data>. 
 
 The structured objects can further have their structure and
 connectivity to other objects verified. This is generally done with
 -the `git-fsck-cache` program, which generates a full dependency graph
 +the `git-fsck-objects` program, which generates a full dependency graph
 of all objects, and verifies their internal consistency (in addition
 to just verifying their superficial consistency through the hash).
 
 The object types in some more detail:
 
 Blob Object
 ~~~~~~~~~~~
 A "blob" object is nothing but a binary blob of data, and doesn't
 refer to anything else.  There is no signature or any other
 verification of the data, so while the object is consistent (it 'is'
 indexed by its sha1 hash, so the data itself is certainly correct), it
 has absolutely no other attributes.  No name associations, no
 permissions.  It is purely a blob of data (i.e. normally "file
 contents").
 
 In particular, since the blob is entirely defined by its data, if two
 files in a directory tree (or in multiple different versions of the
 repository) have the same contents, they will share the same blob
 object. The object is totally independent of its location in the
 directory tree, and renaming a file does not change the object that
 file is associated with in any way.
 
 -A blob is typically created when link:git-update-cache.html[git-update-cache]
 +A blob is typically created when link:git-update-index.html[git-update-index]
 is run, and its data can be accessed by link:git-cat-file.html[git-cat-file].
 
 Tree Object
 ~~~~~~~~~~~
 The next hierarchical object type is the "tree" object.  A tree object
 is a list of mode/name/blob data, sorted by name.  Alternatively, the
 mode data may specify a directory mode, in which case instead of
 naming a blob, that name is associated with another TREE object.
 
 Like the "blob" object, a tree object is uniquely determined by the
 set contents, and so two separate but identical trees will always
 share the exact same object. This is true at all levels, i.e. it's
 true for a "leaf" tree (which does not refer to any other trees, only
 blobs) as well as for a whole subdirectory.
 
 For that reason a "tree" object is just a pure data abstraction: it
 has no history, no signatures, no verification of validity, except
 that since the contents are again protected by the hash itself, we can
 trust that the tree is immutable and its contents never change.
 
 So you can trust the contents of a tree to be valid, the same way you
 can trust the contents of a blob, but you don't know where those
 contents 'came' from.
 
 Side note on trees: since a "tree" object is a sorted list of
 "filename+content", you can create a diff between two trees without
 actually having to unpack two trees.  Just ignore all common parts,
 and your diff will look right.  In other words, you can effectively
 (and efficiently) tell the difference between any two random trees by
 O(n) where "n" is the size of the difference, rather than the size of
 the tree.
 
 Side note 2 on trees: since the name of a "blob" depends entirely and
 exclusively on its contents (i.e. there are no names or permissions
 involved), you can see trivial renames or permission changes by
 noticing that the blob stayed the same.  However, renames with data
 changes need a smarter "diff" implementation.
 
 A tree is created with link:git-write-tree.html[git-write-tree] and
 its data can be accessed by link:git-ls-tree.html[git-ls-tree].
 Two trees can be compared with link:git-diff-tree.html[git-diff-tree].
 
 Commit Object
 ~~~~~~~~~~~~~
 The "commit" object is an object that introduces the notion of
 history into the picture.  In contrast to the other objects, it
 doesn't just describe the physical state of a tree, it describes how
 we got there, and why.
 
 A "commit" is defined by the tree-object that it results in, the
 parent commits (zero, one or more) that led up to that point, and a
 comment on what happened.  Again, a commit is not trusted per se:
 the contents are well-defined and "safe" due to the cryptographically
 strong signatures at all levels, but there is no reason to believe
 that the tree is "good" or that the merge information makes sense.
 The parents do not have to actually have any relationship with the
 result, for example.
 
 Note on commits: unlike real SCM's, commits do not contain
 rename information or file mode change information.  All of that is
 implicit in the trees involved (the result tree, and the result trees
 of the parents), and describing that makes no sense in this idiotic
 file manager.
 
 A commit is created with link:git-commit-tree.html[git-commit-tree] and
 its data can be accessed by link:git-cat-file.html[git-cat-file].
 
 Trust
 ~~~~~
 An aside on the notion of "trust". Trust is really outside the scope
 of "git", but it's worth noting a few things.  First off, since
 everything is hashed with SHA1, you 'can' trust that an object is
 intact and has not been messed with by external sources.  So the name
 of an object uniquely identifies a known state - just not a state that
 you may want to trust.
 
 Furthermore, since the SHA1 signature of a commit refers to the
 SHA1 signatures of the tree it is associated with and the signatures
 of the parent, a single named commit specifies uniquely a whole set
 of history, with full contents.  You can't later fake any step of the
 way once you have the name of a commit.
 
 So to introduce some real trust in the system, the only thing you need
 to do is to digitally sign just 'one' special note, which includes the
 name of a top-level commit.  Your digital signature shows others
 that you trust that commit, and the immutability of the history of
 commits tells others that they can trust the whole history.
 
 In other words, you can easily validate a whole archive by just
 sending out a single email that tells the people the name (SHA1 hash)
 of the top commit, and digitally sign that email using something
 like GPG/PGP.
 
 To assist in this, git also provides the tag object...
 
 Tag Object
 ~~~~~~~~~~
 Git provides the "tag" object to simplify creating, managing and
 exchanging symbolic and signed tokens.  The "tag" object at its
 simplest simply symbolically identifies another object by containing
 the sha1, type and symbolic name.
 
 However it can optionally contain additional signature information
 (which git doesn't care about as long as there's less than 8k of
 it). This can then be verified externally to git.
 
 Note that despite the tag features, "git" itself only handles content
 integrity; the trust framework (and signature provision and
 verification) has to come from outside.
 
 A tag is created with link:git-mktag.html[git-mktag],
 its data can be accessed by link:git-cat-file.html[git-cat-file],
 and the signature can be verified by
 -link:git-verify-tag-script.html[git-verify-tag].
 +link:git-verify-tag.html[git-verify-tag].
 
 
 The "index" aka "Current Directory Cache"
 -----------------------------------------
 The index is a simple binary file, which contains an efficient
 representation of a virtual directory content at some random time.  It
 does so by a simple array that associates a set of names, dates,
 permissions and content (aka "blob") objects together.  The cache is
 always kept ordered by name, and names are unique (with a few very
 specific rules) at any point in time, but the cache has no long-term
 meaning, and can be partially updated at any time.
 
 In particular, the index certainly does not need to be consistent with
 the current directory contents (in fact, most operations will depend on
 different ways to make the index 'not' be consistent with the directory
 hierarchy), but it has three very important attributes:
 
 '(a) it can re-generate the full state it caches (not just the
 directory structure: it contains pointers to the "blob" objects so
 that it can regenerate the data too)'
 
 As a special case, there is a clear and unambiguous one-way mapping
 from a current directory cache to a "tree object", which can be
 efficiently created from just the current directory cache without
 actually looking at any other data.  So a directory cache at any one
 time uniquely specifies one and only one "tree" object (but has
 additional data to make it easy to match up that tree object with what
 has happened in the directory)
 
 '(b) it has efficient methods for finding inconsistencies between that
 cached state ("tree object waiting to be instantiated") and the
 current state.'
 
 '(c) it can additionally efficiently represent information about merge
 conflicts between different tree objects, allowing each pathname to be
 associated with sufficient information about the trees involved that
 you can create a three-way merge between them.'
 
 Those are the three ONLY things that the directory cache does.  It's a
 cache, and the normal operation is to re-generate it completely from a
 known tree object, or update/compare it with a live tree that is being
 developed.  If you blow the directory cache away entirely, you generally
 haven't lost any information as long as you have the name of the tree
 that it described. 
 
 At the same time, the index is at the same time also the
 staging area for creating new trees, and creating a new tree always
 involves a controlled modification of the index file.  In particular,
 the index file can have the representation of an intermediate tree that
 has not yet been instantiated.  So the index can be thought of as a
 write-back cache, which can contain dirty information that has not yet
 been written back to the backing store.
 
 
 
 The Workflow
 ------------
 Generally, all "git" operations work on the index file. Some operations
 work *purely* on the index file (showing the current state of the
 index), but most operations move data to and from the index file. Either
 from the database or from the working directory. Thus there are four
 main combinations: 
 
 1) working directory -> index
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You update the index with information from the working directory with
 -the link:git-update-cache.html[git-update-cache] command.  You
 +the link:git-update-index.html[git-update-index] command.  You
 generally update the index information by just specifying the filename
 you want to update, like so:
 
 -	git-update-cache filename
 +	git-update-index filename
 
 but to avoid common mistakes with filename globbing etc, the command
 will not normally add totally new entries or remove old entries,
 i.e. it will normally just update existing cache entries.
 
 To tell git that yes, you really do realize that certain files no
 longer exist in the archive, or that new files should be added, you
 should use the `--remove` and `--add` flags respectively.
 
 NOTE! A `--remove` flag does 'not' mean that subsequent filenames will
 necessarily be removed: if the files still exist in your directory
 structure, the index will be updated with their new status, not
 removed. The only thing `--remove` means is that update-cache will be
 considering a removed file to be a valid thing, and if the file really
 does not exist any more, it will update the index accordingly.
 
 -As a special case, you can also do `git-update-cache --refresh`, which
 +As a special case, you can also do `git-update-index --refresh`, which
 will refresh the "stat" information of each index to match the current
 stat information. It will 'not' update the object status itself, and
 it will only update the fields that are used to quickly test whether
 an object still matches its old backing store object.
 
 2) index -> object database
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You write your current index file to a "tree" object with the program
 
 	git-write-tree
 
 that doesn't come with any options - it will just write out the
 current index into the set of tree objects that describe that state,
 and it will return the name of the resulting top-level tree. You can
 use that tree to re-generate the index at any time by going in the
 other direction:
 
 3) object database -> index
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You read a "tree" file from the object database, and use that to
 populate (and overwrite - don't do this if your index contains any
 unsaved state that you might want to restore later!) your current
 index.  Normal operation is just
 
 		git-read-tree <sha1 of tree>
 
 and your index file will now be equivalent to the tree that you saved
 earlier. However, that is only your 'index' file: your working
 directory contents have not been modified.
 
 4) index -> working directory
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You update your working directory from the index by "checking out"
 files. This is not a very common operation, since normally you'd just
 keep your files updated, and rather than write to your working
 directory, you'd tell the index files about the changes in your
 -working directory (i.e. `git-update-cache`).
 +working directory (i.e. `git-update-index`).
 
 However, if you decide to jump to a new version, or check out somebody
 else's version, or just restore a previous tree, you'd populate your
 index file with read-tree, and then you need to check out the result
 with
 
 -		git-checkout-cache filename
 +		git-checkout-index filename
 
 or, if you want to check out all of the index, use `-a`.
 
 -NOTE! git-checkout-cache normally refuses to overwrite old files, so
 +NOTE! git-checkout-index normally refuses to overwrite old files, so
 if you have an old version of the tree already checked out, you will
 need to use the "-f" flag ('before' the "-a" flag or the filename) to
 'force' the checkout.
 
 
 Finally, there are a few odds and ends which are not purely moving
 from one representation to the other:
 
 5) Tying it all together
 ~~~~~~~~~~~~~~~~~~~~~~~~
 To commit a tree you have instantiated with "git-write-tree", you'd
 create a "commit" object that refers to that tree and the history
 behind it - most notably the "parent" commits that preceded it in
 history.
 
 Normally a "commit" has one parent: the previous state of the tree
 before a certain change was made. However, sometimes it can have two
 or more parent commits, in which case we call it a "merge", due to the
 fact that such a commit brings together ("merges") two or more
 previous states represented by other commits.
 
 In other words, while a "tree" represents a particular directory state
 of a working directory, a "commit" represents that state in "time",
 and explains how we got there.
 
 You create a commit object by giving it the tree that describes the
 state at the time of the commit, and a list of parents:
 
 	git-commit-tree <tree> -p <parent> [-p <parent2> ..]
 
 and then giving the reason for the commit on stdin (either through
 redirection from a pipe or file, or by just typing it at the tty).
 
 git-commit-tree will return the name of the object that represents
 that commit, and you should save it away for later use. Normally,
 you'd commit a new `HEAD` state, and while git doesn't care where you
 save the note about that state, in practice we tend to just write the
 result to the file `.git/HEAD`, so that we can always see what the
 last committed state was.
 
 6) Examining the data
 ~~~~~~~~~~~~~~~~~~~~~
 
 You can examine the data represented in the object database and the
 index with various helper tools. For every object, you can use
 link:git-cat-file.html[git-cat-file] to examine details about the
 object:
 
 		git-cat-file -t <objectname>
 
 shows the type of the object, and once you have the type (which is
 usually implicit in where you find the object), you can use
 
 		git-cat-file blob|tree|commit|tag <objectname>
 
 to show its contents. NOTE! Trees have binary content, and as a result
 there is a special helper for showing that content, called
 `git-ls-tree`, which turns the binary content into a more easily
 readable form.
 
 It's especially instructive to look at "commit" objects, since those
 tend to be small and fairly self-explanatory. In particular, if you
 follow the convention of having the top commit name in `.git/HEAD`,
 you can do
 
 		git-cat-file commit $(cat .git/HEAD)
 
 to see what the top commit was.
 
 7) Merging multiple trees
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Git helps you do a three-way merge, which you can expand to n-way by
 repeating the merge procedure arbitrary times until you finally
 "commit" the state.  The normal situation is that you'd only do one
 three-way merge (two parents), and commit it, but if you like to, you
 can do multiple parents in one go.
 
 To do a three-way merge, you need the two sets of "commit" objects
 that you want to merge, use those to find the closest common parent (a
 third "commit" object), and then use those commit objects to find the
 state of the directory ("tree" object) at these points.
 
 To get the "base" for the merge, you first look up the common parent
 of two commits with
 
 		git-merge-base <commit1> <commit2>
 
 which will return you the commit they are both based on.  You should
 now look up the "tree" objects of those commits, which you can easily
 do with (for example)
 
 		git-cat-file commit <commitname> | head -1
 
 since the tree object information is always the first line in a commit
 object.
 
 Once you know the three trees you are going to merge (the one
 "original" tree, aka the common case, and the two "result" trees, aka
 the branches you want to merge), you do a "merge" read into the
 index. This will complain if it has to throw away your old index contents, so you should
 make sure that you've committed those - in fact you would normally
 always do a merge against your last commit (which should thus match
 what you have in your current index anyway).
 
 To do the merge, do
 
 		git-read-tree -m -u <origtree> <yourtree> <targettree>
 
 which will do all trivial merge operations for you directly in the
 index file, and you can just write the result out with
 `git-write-tree`.
 
 Historical note.  We did not have `-u` facility when this
 section was first written, so we used to warn that
 the merge is done in the index file, not in your
 working directory, and your working directory will no longer match your
 index.
 
 
 8) Merging multiple trees, continued
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Sadly, many merges aren't trivial. If there are files that have
 been added.moved or removed, or if both branches have modified the
 same file, you will be left with an index tree that contains "merge
 entries" in it. Such an index tree can 'NOT' be written out to a tree
 object, and you will have to resolve any such merge clashes using
 other tools before you can write out the result.
 
 You can examine such index state with `git-ls-files --unmerged`
 command.  An example:
 
 ------------------------------------------------
 $ git-read-tree -m $orig HEAD $target
 $ git-ls-files --unmerged
 100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1	hello.c
 100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2	hello.c
 100644 cc44c73eb783565da5831b4d820c962954019b69 3	hello.c
 ------------------------------------------------
 
 Each line of the `git-ls-files --unmerged` output begins with
 the blob mode bits, blob SHA1, 'stage number', and the
 filename.  The 'stage number' is git's way to say which tree it
 came from: stage 1 corresponds to `$orig` tree, stage 2 `HEAD`
 tree, and stage3 `$target` tree.
 
 Earlier we said that trivial merges are done inside
 `git-read-tree -m`.  For example, if the file did not change
 from `$orig` to `HEAD` nor `$target`, or if the file changed
 from `$orig` to `HEAD` and `$orig` to `$target` the same way,
 obviously the final outcome is what is in `HEAD`.  What the
 above example shows is that file `hello.c` was changed from
 `$orig` to `HEAD` and `$orig` to `$target` in a different way.
 You could resolve this by running your favorite 3-way merge
 program, e.g.  `diff3` or `merge`, on the blob objects from
 these three stages yourself, like this:
 
 ------------------------------------------------
 $ git-cat-file blob 263414f... >hello.c~1
 $ git-cat-file blob 06fa6a2... >hello.c~2
 $ git-cat-file blob cc44c73... >hello.c~3
 $ merge hello.c~2 hello.c~1 hello.c~3
 ------------------------------------------------
 
 This would leave the merge result in `hello.c~2` file, along
 with conflict markers if there are conflicts.  After verifying
 the merge result makes sense, you can tell git what the final
 merge result for this file is by:
 
 	mv -f hello.c~2 hello.c
 -	git-update-cache hello.c
 +	git-update-index hello.c
 
 -When a path is in unmerged state, running `git-update-cache` for
 +When a path is in unmerged state, running `git-update-index` for
 that path tells git to mark the path resolved.
 
 The above is the description of a git merge at the lowest level,
 to help you understand what conceptually happens under the hood.
 In practice, nobody, not even git itself, uses three `git-cat-file`
 -for this.  There is `git-merge-cache` program that extracts the
 +for this.  There is `git-merge-index` program that extracts the
 stages to temporary files and calls a `merge` script on it
 
 -	git-merge-cache git-merge-one-file-script hello.c
 +	git-merge-index git-merge-one-file hello.c
 
 and that is what higher level `git resolve` is implemented with.

 commit 8db9307c9ca143fedaa972236054a5783c40bd37
 Author: Junio C Hamano <[email protected]>
 Date:   Tue Aug 30 13:51:01 2005 -0700

    Documentaion updates.
    
    Mostly making the formatted html prettier.
    
    Signed-off-by: Junio C Hamano <[email protected]>
    (cherry picked from 7adf1f15ebe074d4767df941817a6cf86d8e2533 commit)

 diff --git a/README b/README
 --- a/README
 +++ b/README
 @@ -17,466 +17,530 @@
 This is a stupid (but extremely fast) directory content manager.  It
 -doesn't do a whole lot, but what it _does_ do is track directory
 +doesn't do a whole lot, but what it 'does' do is track directory
 contents efficiently. 
 
 There are two object abstractions: the "object database", and the
 "current directory cache" aka "index".
 
 The Object Database
 ~~~~~~~~~~~~~~~~~~~
 The object database is literally just a content-addressable collection
 of objects.  All objects are named by their content, which is
 approximated by the SHA1 hash of the object itself.  Objects may refer
 to other objects (by referencing their SHA1 hash), and so you can
 build up a hierarchy of objects.
 
 All objects have a statically determined "type" aka "tag", which is
 determined at object creation time, and which identifies the format of
 the object (i.e. how it is used, and how it can refer to other
 objects).  There are currently four different object types: "blob",
 "tree", "commit" and "tag".
 
 A "blob" object cannot refer to any other object, and is, like the tag
 implies, a pure storage object containing some user data.  It is used to
 actually store the file data, i.e. a blob object is associated with some
 particular version of some file. 
 
 A "tree" object is an object that ties one or more "blob" objects into a
 directory structure. In addition, a tree object can refer to other tree
 objects, thus creating a directory hierarchy. 
 
 A "commit" object ties such directory hierarchies together into
 a DAG of revisions - each "commit" is associated with exactly one tree
 (the directory hierarchy at the time of the commit). In addition, a
 "commit" refers to one or more "parent" commit objects that describe the
 history of how we arrived at that directory hierarchy.
 
 As a special case, a commit object with no parents is called the "root"
 object, and is the point of an initial project commit.  Each project
 must have at least one root, and while you can tie several different
 root objects together into one project by creating a commit object which
 has two or more separate roots as its ultimate parents, that's probably
 just going to confuse people.  So aim for the notion of "one root object
 per project", even if git itself does not enforce that. 
 
 A "tag" object symbolically identifies and can be used to sign other
 objects. It contains the identifier and type of another object, a
 symbolic name (of course!) and, optionally, a signature.
 
 Regardless of object type, all objects share the following
 characteristics: they are all deflated with zlib, and have a header
 that not only specifies their tag, but also provides size information
 about the data in the object.  It's worth noting that the SHA1 hash
 -that is used to name the object is the hash of the original data.
 +that is used to name the object is the hash of the original data
 +plus this header, so `sha1sum` 'file' does not match the object name
 +for 'file'.
 (Historical note: in the dawn of the age of git the hash
 -was the sha1 of the _compressed_ object)
 +was the sha1 of the 'compressed' object.)
 
 As a result, the general consistency of an object can always be tested
 independently of the contents or the type of the object: all objects can
 be validated by verifying that (a) their hashes match the content of the
 file and (b) the object successfully inflates to a stream of bytes that
 forms a sequence of <ascii tag without space> + <space> + <ascii decimal
 size> + <byte\0> + <binary object data>. 
 
 The structured objects can further have their structure and
 connectivity to other objects verified. This is generally done with
 -the "git-fsck-cache" program, which generates a full dependency graph
 +the `git-fsck-cache` program, which generates a full dependency graph
 of all objects, and verifies their internal consistency (in addition
 to just verifying their superficial consistency through the hash).
 
 The object types in some more detail:
 
 Blob Object
 ~~~~~~~~~~~
 A "blob" object is nothing but a binary blob of data, and doesn't
 refer to anything else.  There is no signature or any other
 -verification of the data, so while the object is consistent (it _is_
 +verification of the data, so while the object is consistent (it 'is'
 indexed by its sha1 hash, so the data itself is certainly correct), it
 has absolutely no other attributes.  No name associations, no
 permissions.  It is purely a blob of data (i.e. normally "file
 contents").
 
 In particular, since the blob is entirely defined by its data, if two
 files in a directory tree (or in multiple different versions of the
 repository) have the same contents, they will share the same blob
 object. The object is totally independent of its location in the
 directory tree, and renaming a file does not change the object that
 file is associated with in any way.
 
 A blob is typically created when link:git-update-cache.html[git-update-cache]
 is run, and its data can be accessed by link:git-cat-file.html[git-cat-file].
 
 Tree Object
 ~~~~~~~~~~~
 The next hierarchical object type is the "tree" object.  A tree object
 is a list of mode/name/blob data, sorted by name.  Alternatively, the
 mode data may specify a directory mode, in which case instead of
 naming a blob, that name is associated with another TREE object.
 
 Like the "blob" object, a tree object is uniquely determined by the
 set contents, and so two separate but identical trees will always
 share the exact same object. This is true at all levels, i.e. it's
 true for a "leaf" tree (which does not refer to any other trees, only
 blobs) as well as for a whole subdirectory.
 
 For that reason a "tree" object is just a pure data abstraction: it
 has no history, no signatures, no verification of validity, except
 that since the contents are again protected by the hash itself, we can
 trust that the tree is immutable and its contents never change.
 
 So you can trust the contents of a tree to be valid, the same way you
 can trust the contents of a blob, but you don't know where those
 -contents _came_ from.
 +contents 'came' from.
 
 Side note on trees: since a "tree" object is a sorted list of
 "filename+content", you can create a diff between two trees without
 actually having to unpack two trees.  Just ignore all common parts,
 and your diff will look right.  In other words, you can effectively
 (and efficiently) tell the difference between any two random trees by
 O(n) where "n" is the size of the difference, rather than the size of
 the tree.
 
 Side note 2 on trees: since the name of a "blob" depends entirely and
 exclusively on its contents (i.e. there are no names or permissions
 involved), you can see trivial renames or permission changes by
 noticing that the blob stayed the same.  However, renames with data
 changes need a smarter "diff" implementation.
 
 A tree is created with link:git-write-tree.html[git-write-tree] and
 -its data can be accessed by link:git-ls-tree.html[git-ls-tree]
 +its data can be accessed by link:git-ls-tree.html[git-ls-tree].
 +Two trees can be compared with link:git-diff-tree.html[git-diff-tree].
 
 Commit Object
 ~~~~~~~~~~~~~
 The "commit" object is an object that introduces the notion of
 history into the picture.  In contrast to the other objects, it
 doesn't just describe the physical state of a tree, it describes how
 we got there, and why.
 
 A "commit" is defined by the tree-object that it results in, the
 parent commits (zero, one or more) that led up to that point, and a
 comment on what happened.  Again, a commit is not trusted per se:
 the contents are well-defined and "safe" due to the cryptographically
 strong signatures at all levels, but there is no reason to believe
 that the tree is "good" or that the merge information makes sense.
 The parents do not have to actually have any relationship with the
 result, for example.
 
 Note on commits: unlike real SCM's, commits do not contain
 -rename information or file mode chane information.  All of that is
 +rename information or file mode change information.  All of that is
 implicit in the trees involved (the result tree, and the result trees
 of the parents), and describing that makes no sense in this idiotic
 file manager.
 
 A commit is created with link:git-commit-tree.html[git-commit-tree] and
 -its data can be accessed by link:git-cat-file.html[git-cat-file]
 +its data can be accessed by link:git-cat-file.html[git-cat-file].
 
 Trust
 ~~~~~
 An aside on the notion of "trust". Trust is really outside the scope
 of "git", but it's worth noting a few things.  First off, since
 -everything is hashed with SHA1, you _can_ trust that an object is
 +everything is hashed with SHA1, you 'can' trust that an object is
 intact and has not been messed with by external sources.  So the name
 of an object uniquely identifies a known state - just not a state that
 you may want to trust.
 
 Furthermore, since the SHA1 signature of a commit refers to the
 SHA1 signatures of the tree it is associated with and the signatures
 of the parent, a single named commit specifies uniquely a whole set
 of history, with full contents.  You can't later fake any step of the
 way once you have the name of a commit.
 
 So to introduce some real trust in the system, the only thing you need
 -to do is to digitally sign just _one_ special note, which includes the
 +to do is to digitally sign just 'one' special note, which includes the
 name of a top-level commit.  Your digital signature shows others
 that you trust that commit, and the immutability of the history of
 commits tells others that they can trust the whole history.
 
 In other words, you can easily validate a whole archive by just
 sending out a single email that tells the people the name (SHA1 hash)
 of the top commit, and digitally sign that email using something
 like GPG/PGP.
 
 To assist in this, git also provides the tag object...
 
 Tag Object
 ~~~~~~~~~~
 Git provides the "tag" object to simplify creating, managing and
 exchanging symbolic and signed tokens.  The "tag" object at its
 simplest simply symbolically identifies another object by containing
 the sha1, type and symbolic name.
 
 However it can optionally contain additional signature information
 (which git doesn't care about as long as there's less than 8k of
 it). This can then be verified externally to git.
 
 Note that despite the tag features, "git" itself only handles content
 integrity; the trust framework (and signature provision and
 verification) has to come from outside.
 
 -A tag is created with link:git-mktag.html[git-mktag] and
 -its data can be accessed by link:git-cat-file.html[git-cat-file]
 +A tag is created with link:git-mktag.html[git-mktag],
 +its data can be accessed by link:git-cat-file.html[git-cat-file],
 +and the signature can be verified by
 +link:git-verify-tag-script.html[git-verify-tag].
 
 
 The "index" aka "Current Directory Cache"
 -----------------------------------------
 The index is a simple binary file, which contains an efficient
 representation of a virtual directory content at some random time.  It
 does so by a simple array that associates a set of names, dates,
 permissions and content (aka "blob") objects together.  The cache is
 always kept ordered by name, and names are unique (with a few very
 specific rules) at any point in time, but the cache has no long-term
 meaning, and can be partially updated at any time.
 
 In particular, the index certainly does not need to be consistent with
 the current directory contents (in fact, most operations will depend on
 -different ways to make the index _not_ be consistent with the directory
 +different ways to make the index 'not' be consistent with the directory
 hierarchy), but it has three very important attributes:
 
 '(a) it can re-generate the full state it caches (not just the
 directory structure: it contains pointers to the "blob" objects so
 that it can regenerate the data too)'
 
 As a special case, there is a clear and unambiguous one-way mapping
 from a current directory cache to a "tree object", which can be
 efficiently created from just the current directory cache without
 actually looking at any other data.  So a directory cache at any one
 time uniquely specifies one and only one "tree" object (but has
 additional data to make it easy to match up that tree object with what
 has happened in the directory)
 
 '(b) it has efficient methods for finding inconsistencies between that
 cached state ("tree object waiting to be instantiated") and the
 current state.'
 
 '(c) it can additionally efficiently represent information about merge
 conflicts between different tree objects, allowing each pathname to be
 associated with sufficient information about the trees involved that
 you can create a three-way merge between them.'
 
 Those are the three ONLY things that the directory cache does.  It's a
 cache, and the normal operation is to re-generate it completely from a
 known tree object, or update/compare it with a live tree that is being
 developed.  If you blow the directory cache away entirely, you generally
 haven't lost any information as long as you have the name of the tree
 that it described. 
 
 -At the same time, the directory index is at the same time also the
 +At the same time, the index is at the same time also the
 staging area for creating new trees, and creating a new tree always
 involves a controlled modification of the index file.  In particular,
 the index file can have the representation of an intermediate tree that
 has not yet been instantiated.  So the index can be thought of as a
 write-back cache, which can contain dirty information that has not yet
 been written back to the backing store.
 
 
 
 The Workflow
 ------------
 Generally, all "git" operations work on the index file. Some operations
 work *purely* on the index file (showing the current state of the
 index), but most operations move data to and from the index file. Either
 from the database or from the working directory. Thus there are four
 main combinations: 
 
 1) working directory -> index
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You update the index with information from the working directory with
 the link:git-update-cache.html[git-update-cache] command.  You
 generally update the index information by just specifying the filename
 you want to update, like so:
 
 	git-update-cache filename
 
 but to avoid common mistakes with filename globbing etc, the command
 will not normally add totally new entries or remove old entries,
 i.e. it will normally just update existing cache entries.
 
 To tell git that yes, you really do realize that certain files no
 longer exist in the archive, or that new files should be added, you
 -should use the "--remove" and "--add" flags respectively.
 +should use the `--remove` and `--add` flags respectively.
 
 -NOTE! A "--remove" flag does _not_ mean that subsequent filenames will
 +NOTE! A `--remove` flag does 'not' mean that subsequent filenames will
 necessarily be removed: if the files still exist in your directory
 structure, the index will be updated with their new status, not
 -removed. The only thing "--remove" means is that update-cache will be
 +removed. The only thing `--remove` means is that update-cache will be
 considering a removed file to be a valid thing, and if the file really
 does not exist any more, it will update the index accordingly.
 
 -As a special case, you can also do "git-update-cache --refresh", which
 +As a special case, you can also do `git-update-cache --refresh`, which
 will refresh the "stat" information of each index to match the current
 -stat information. It will _not_ update the object status itself, and
 +stat information. It will 'not' update the object status itself, and
 it will only update the fields that are used to quickly test whether
 an object still matches its old backing store object.
 
 2) index -> object database
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You write your current index file to a "tree" object with the program
 
 	git-write-tree
 
 that doesn't come with any options - it will just write out the
 current index into the set of tree objects that describe that state,
 and it will return the name of the resulting top-level tree. You can
 use that tree to re-generate the index at any time by going in the
 other direction:
 
 3) object database -> index
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You read a "tree" file from the object database, and use that to
 populate (and overwrite - don't do this if your index contains any
 unsaved state that you might want to restore later!) your current
 index.  Normal operation is just
 
 		git-read-tree <sha1 of tree>
 
 and your index file will now be equivalent to the tree that you saved
 -earlier. However, that is only your _index_ file: your working
 +earlier. However, that is only your 'index' file: your working
 directory contents have not been modified.
 
 4) index -> working directory
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You update your working directory from the index by "checking out"
 files. This is not a very common operation, since normally you'd just
 keep your files updated, and rather than write to your working
 directory, you'd tell the index files about the changes in your
 -working directory (i.e. "git-update-cache").
 +working directory (i.e. `git-update-cache`).
 
 However, if you decide to jump to a new version, or check out somebody
 else's version, or just restore a previous tree, you'd populate your
 index file with read-tree, and then you need to check out the result
 with
 +
 		git-checkout-cache filename
 
 -or, if you want to check out all of the index, use "-a".
 +or, if you want to check out all of the index, use `-a`.
 
 NOTE! git-checkout-cache normally refuses to overwrite old files, so
 if you have an old version of the tree already checked out, you will
 -need to use the "-f" flag (_before_ the "-a" flag or the filename) to
 -_force_ the checkout.
 +need to use the "-f" flag ('before' the "-a" flag or the filename) to
 +'force' the checkout.
 
 
 Finally, there are a few odds and ends which are not purely moving
 from one representation to the other:
 
 5) Tying it all together
 ~~~~~~~~~~~~~~~~~~~~~~~~
 To commit a tree you have instantiated with "git-write-tree", you'd
 create a "commit" object that refers to that tree and the history
 behind it - most notably the "parent" commits that preceded it in
 history.
 
 Normally a "commit" has one parent: the previous state of the tree
 before a certain change was made. However, sometimes it can have two
 or more parent commits, in which case we call it a "merge", due to the
 fact that such a commit brings together ("merges") two or more
 previous states represented by other commits.
 
 In other words, while a "tree" represents a particular directory state
 of a working directory, a "commit" represents that state in "time",
 and explains how we got there.
 
 You create a commit object by giving it the tree that describes the
 state at the time of the commit, and a list of parents:
 
 	git-commit-tree <tree> -p <parent> [-p <parent2> ..]
 
 and then giving the reason for the commit on stdin (either through
 redirection from a pipe or file, or by just typing it at the tty).
 
 git-commit-tree will return the name of the object that represents
 that commit, and you should save it away for later use. Normally,
 -you'd commit a new "HEAD" state, and while git doesn't care where you
 +you'd commit a new `HEAD` state, and while git doesn't care where you
 save the note about that state, in practice we tend to just write the
 -result to the file ".git/HEAD", so that we can always see what the
 +result to the file `.git/HEAD`, so that we can always see what the
 last committed state was.
 
 6) Examining the data
 ~~~~~~~~~~~~~~~~~~~~~
 
 You can examine the data represented in the object database and the
 index with various helper tools. For every object, you can use
 link:git-cat-file.html[git-cat-file] to examine details about the
 object:
 
 		git-cat-file -t <objectname>
 
 shows the type of the object, and once you have the type (which is
 usually implicit in where you find the object), you can use
 
 -		git-cat-file blob|tree|commit <objectname>
 +		git-cat-file blob|tree|commit|tag <objectname>
 
 to show its contents. NOTE! Trees have binary content, and as a result
 there is a special helper for showing that content, called
 -"git-ls-tree", which turns the binary content into a more easily
 +`git-ls-tree`, which turns the binary content into a more easily
 readable form.
 
 It's especially instructive to look at "commit" objects, since those
 tend to be small and fairly self-explanatory. In particular, if you
 -follow the convention of having the top commit name in ".git/HEAD",
 +follow the convention of having the top commit name in `.git/HEAD`,
 you can do
 
 		git-cat-file commit $(cat .git/HEAD)
 
 to see what the top commit was.
 
 7) Merging multiple trees
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Git helps you do a three-way merge, which you can expand to n-way by
 repeating the merge procedure arbitrary times until you finally
 "commit" the state.  The normal situation is that you'd only do one
 three-way merge (two parents), and commit it, but if you like to, you
 can do multiple parents in one go.
 
 To do a three-way merge, you need the two sets of "commit" objects
 that you want to merge, use those to find the closest common parent (a
 third "commit" object), and then use those commit objects to find the
 state of the directory ("tree" object) at these points.
 
 To get the "base" for the merge, you first look up the common parent
 of two commits with
 
 		git-merge-base <commit1> <commit2>
 
 which will return you the commit they are both based on.  You should
 now look up the "tree" objects of those commits, which you can easily
 do with (for example)
 
 		git-cat-file commit <commitname> | head -1
 
 since the tree object information is always the first line in a commit
 object.
 
 Once you know the three trees you are going to merge (the one
 "original" tree, aka the common case, and the two "result" trees, aka
 the branches you want to merge), you do a "merge" read into the
 -index. This will throw away your old index contents, so you should
 +index. This will complain if it has to throw away your old index contents, so you should
 make sure that you've committed those - in fact you would normally
 always do a merge against your last commit (which should thus match
 what you have in your current index anyway).
 
 To do the merge, do
 
 -		git-read-tree -m <origtree> <target1tree> <target2tree>
 +		git-read-tree -m -u <origtree> <yourtree> <targettree>
 
 which will do all trivial merge operations for you directly in the
 index file, and you can just write the result out with
 -"git-write-tree".
 +`git-write-tree`.
 +
 +Historical note.  We did not have `-u` facility when this
 +section was first written, so we used to warn that
 +the merge is done in the index file, not in your
 +working directory, and your working directory will no longer match your
 +index.
 
 -NOTE! Because the merge is done in the index file, and not in your
 -working directory, your working directory will no longer match your
 -index. You can use "git-checkout-cache -f -a" to make the effect of
 -the merge be seen in your working directory.
 
 -NOTE2! Sadly, many merges aren't trivial. If there are files that have
 +8) Merging multiple trees, continued
 +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 +
 +Sadly, many merges aren't trivial. If there are files that have
 been added.moved or removed, or if both branches have modified the
 same file, you will be left with an index tree that contains "merge
 -entries" in it. Such an index tree can _NOT_ be written out to a tree
 +entries" in it. Such an index tree can 'NOT' be written out to a tree
 object, and you will have to resolve any such merge clashes using
 other tools before you can write out the result.
 
 -
 -[ fixme: talk about resolving merges here ]
 +You can examine such index state with `git-ls-files --unmerged`
 +command.  An example:
 +
 +------------------------------------------------
 +$ git-read-tree -m $orig HEAD $target
 +$ git-ls-files --unmerged
 +100644 263414f423d0e4d70dae8fe53fa34614ff3e2860 1	hello.c
 +100644 06fa6a24256dc7e560efa5687fa84b51f0263c3a 2	hello.c
 +100644 cc44c73eb783565da5831b4d820c962954019b69 3	hello.c
 +------------------------------------------------
 +
 +Each line of the `git-ls-files --unmerged` output begins with
 +the blob mode bits, blob SHA1, 'stage number', and the
 +filename.  The 'stage number' is git's way to say which tree it
 +came from: stage 1 corresponds to `$orig` tree, stage 2 `HEAD`
 +tree, and stage3 `$target` tree.
 +
 +Earlier we said that trivial merges are done inside
 +`git-read-tree -m`.  For example, if the file did not change
 +from `$orig` to `HEAD` nor `$target`, or if the file changed
 +from `$orig` to `HEAD` and `$orig` to `$target` the same way,
 +obviously the final outcome is what is in `HEAD`.  What the
 +above example shows is that file `hello.c` was changed from
 +`$orig` to `HEAD` and `$orig` to `$target` in a different way.
 +You could resolve this by running your favorite 3-way merge
 +program, e.g.  `diff3` or `merge`, on the blob objects from
 +these three stages yourself, like this:
 +
 +------------------------------------------------
 +$ git-cat-file blob 263414f... >hello.c~1
 +$ git-cat-file blob 06fa6a2... >hello.c~2
 +$ git-cat-file blob cc44c73... >hello.c~3
 +$ merge hello.c~2 hello.c~1 hello.c~3
 +------------------------------------------------
 +
 +This would leave the merge result in `hello.c~2` file, along
 +with conflict markers if there are conflicts.  After verifying
 +the merge result makes sense, you can tell git what the final
 +merge result for this file is by:
 +
 +	mv -f hello.c~2 hello.c
 +	git-update-cache hello.c
 +
 +When a path is in unmerged state, running `git-update-cache` for
 +that path tells git to mark the path resolved.
 +
 +The above is the description of a git merge at the lowest level,
 +to help you understand what conceptually happens under the hood.
 +In practice, nobody, not even git itself, uses three `git-cat-file`
 +for this.  There is `git-merge-cache` program that extracts the
 +stages to temporary files and calls a `merge` script on it
 +
 +	git-merge-cache git-merge-one-file-script hello.c
 +
 +and that is what higher level `git resolve` is implemented with.

 commit cdacb6208fdb6779b21350644d61bd90e63db3d5
 Author: Greg Louis <[email protected]>
 Date:   Wed Aug 17 12:37:04 2005 -0400

    [PATCH] use it's and its correctly in documentation
    
    At one place in Documentation/tutorial.txt and several in the base
    README, its was wrongly used in place of it's or vice versa.  One
    instance remains somewhere in Documentation/howto/, which I didn't
    correct because it's in a quotation.
    
    Signed-off-by: Greg Louis <[email protected]>
    Signed-off-by: Junio C Hamano <[email protected]>

 diff --git a/README b/README
 --- a/README
 +++ b/README
 @@ -17,466 +17,466 @@
 This is a stupid (but extremely fast) directory content manager.  It
 doesn't do a whole lot, but what it _does_ do is track directory
 contents efficiently. 
 
 There are two object abstractions: the "object database", and the
 "current directory cache" aka "index".
 
 The Object Database
 ~~~~~~~~~~~~~~~~~~~
 The object database is literally just a content-addressable collection
 of objects.  All objects are named by their content, which is
 approximated by the SHA1 hash of the object itself.  Objects may refer
 to other objects (by referencing their SHA1 hash), and so you can
 build up a hierarchy of objects.
 
 All objects have a statically determined "type" aka "tag", which is
 determined at object creation time, and which identifies the format of
 the object (i.e. how it is used, and how it can refer to other
 objects).  There are currently four different object types: "blob",
 "tree", "commit" and "tag".
 
 A "blob" object cannot refer to any other object, and is, like the tag
 implies, a pure storage object containing some user data.  It is used to
 actually store the file data, i.e. a blob object is associated with some
 particular version of some file. 
 
 A "tree" object is an object that ties one or more "blob" objects into a
 directory structure. In addition, a tree object can refer to other tree
 objects, thus creating a directory hierarchy. 
 
 A "commit" object ties such directory hierarchies together into
 a DAG of revisions - each "commit" is associated with exactly one tree
 (the directory hierarchy at the time of the commit). In addition, a
 "commit" refers to one or more "parent" commit objects that describe the
 history of how we arrived at that directory hierarchy.
 
 As a special case, a commit object with no parents is called the "root"
 object, and is the point of an initial project commit.  Each project
 must have at least one root, and while you can tie several different
 root objects together into one project by creating a commit object which
 has two or more separate roots as its ultimate parents, that's probably
 just going to confuse people.  So aim for the notion of "one root object
 per project", even if git itself does not enforce that. 
 
 A "tag" object symbolically identifies and can be used to sign other
 objects. It contains the identifier and type of another object, a
 symbolic name (of course!) and, optionally, a signature.
 
 Regardless of object type, all objects share the following
 characteristics: they are all deflated with zlib, and have a header
 that not only specifies their tag, but also provides size information
 about the data in the object.  It's worth noting that the SHA1 hash
 that is used to name the object is the hash of the original data.
 (Historical note: in the dawn of the age of git the hash
 was the sha1 of the _compressed_ object)
 
 As a result, the general consistency of an object can always be tested
 independently of the contents or the type of the object: all objects can
 be validated by verifying that (a) their hashes match the content of the
 file and (b) the object successfully inflates to a stream of bytes that
 forms a sequence of <ascii tag without space> + <space> + <ascii decimal
 size> + <byte\0> + <binary object data>. 
 
 The structured objects can further have their structure and
 connectivity to other objects verified. This is generally done with
 the "git-fsck-cache" program, which generates a full dependency graph
 of all objects, and verifies their internal consistency (in addition
 to just verifying their superficial consistency through the hash).
 
 The object types in some more detail:
 
 Blob Object
 ~~~~~~~~~~~
 A "blob" object is nothing but a binary blob of data, and doesn't
 refer to anything else.  There is no signature or any other
 verification of the data, so while the object is consistent (it _is_
 indexed by its sha1 hash, so the data itself is certainly correct), it
 has absolutely no other attributes.  No name associations, no
 permissions.  It is purely a blob of data (i.e. normally "file
 contents").
 
 In particular, since the blob is entirely defined by its data, if two
 files in a directory tree (or in multiple different versions of the
 repository) have the same contents, they will share the same blob
 -object. The object is totally independent of it's location in the
 +object. The object is totally independent of its location in the
 directory tree, and renaming a file does not change the object that
 file is associated with in any way.
 
 A blob is typically created when link:git-update-cache.html[git-update-cache]
 -is run, and it's data can be accessed by link:git-cat-file.html[git-cat-file].
 +is run, and its data can be accessed by link:git-cat-file.html[git-cat-file].
 
 Tree Object
 ~~~~~~~~~~~
 The next hierarchical object type is the "tree" object.  A tree object
 is a list of mode/name/blob data, sorted by name.  Alternatively, the
 mode data may specify a directory mode, in which case instead of
 naming a blob, that name is associated with another TREE object.
 
 Like the "blob" object, a tree object is uniquely determined by the
 set contents, and so two separate but identical trees will always
 share the exact same object. This is true at all levels, i.e. it's
 true for a "leaf" tree (which does not refer to any other trees, only
 blobs) as well as for a whole subdirectory.
 
 For that reason a "tree" object is just a pure data abstraction: it
 has no history, no signatures, no verification of validity, except
 that since the contents are again protected by the hash itself, we can
 trust that the tree is immutable and its contents never change.
 
 So you can trust the contents of a tree to be valid, the same way you
 can trust the contents of a blob, but you don't know where those
 contents _came_ from.
 
 Side note on trees: since a "tree" object is a sorted list of
 "filename+content", you can create a diff between two trees without
 actually having to unpack two trees.  Just ignore all common parts,
 and your diff will look right.  In other words, you can effectively
 (and efficiently) tell the difference between any two random trees by
 O(n) where "n" is the size of the difference, rather than the size of
 the tree.
 
 Side note 2 on trees: since the name of a "blob" depends entirely and
 exclusively on its contents (i.e. there are no names or permissions
 involved), you can see trivial renames or permission changes by
 noticing that the blob stayed the same.  However, renames with data
 changes need a smarter "diff" implementation.
 
 A tree is created with link:git-write-tree.html[git-write-tree] and
 -it's data can be accessed by link:git-ls-tree.html[git-ls-tree]
 +its data can be accessed by link:git-ls-tree.html[git-ls-tree]
 
 Commit Object
 ~~~~~~~~~~~~~
 The "commit" object is an object that introduces the notion of
 history into the picture.  In contrast to the other objects, it
 doesn't just describe the physical state of a tree, it describes how
 we got there, and why.
 
 A "commit" is defined by the tree-object that it results in, the
 parent commits (zero, one or more) that led up to that point, and a
 comment on what happened.  Again, a commit is not trusted per se:
 the contents are well-defined and "safe" due to the cryptographically
 strong signatures at all levels, but there is no reason to believe
 that the tree is "good" or that the merge information makes sense.
 The parents do not have to actually have any relationship with the
 result, for example.
 
 Note on commits: unlike real SCM's, commits do not contain
 rename information or file mode chane information.  All of that is
 implicit in the trees involved (the result tree, and the result trees
 of the parents), and describing that makes no sense in this idiotic
 file manager.
 
 A commit is created with link:git-commit-tree.html[git-commit-tree] and
 -it's data can be accessed by link:git-cat-file.html[git-cat-file]
 +its data can be accessed by link:git-cat-file.html[git-cat-file]
 
 Trust
 ~~~~~
 An aside on the notion of "trust". Trust is really outside the scope
 of "git", but it's worth noting a few things.  First off, since
 everything is hashed with SHA1, you _can_ trust that an object is
 intact and has not been messed with by external sources.  So the name
 of an object uniquely identifies a known state - just not a state that
 you may want to trust.
 
 Furthermore, since the SHA1 signature of a commit refers to the
 SHA1 signatures of the tree it is associated with and the signatures
 of the parent, a single named commit specifies uniquely a whole set
 of history, with full contents.  You can't later fake any step of the
 way once you have the name of a commit.
 
 So to introduce some real trust in the system, the only thing you need
 to do is to digitally sign just _one_ special note, which includes the
 name of a top-level commit.  Your digital signature shows others
 that you trust that commit, and the immutability of the history of
 commits tells others that they can trust the whole history.
 
 In other words, you can easily validate a whole archive by just
 sending out a single email that tells the people the name (SHA1 hash)
 of the top commit, and digitally sign that email using something
 like GPG/PGP.
 
 To assist in this, git also provides the tag object...
 
 Tag Object
 ~~~~~~~~~~
 Git provides the "tag" object to simplify creating, managing and
 exchanging symbolic and signed tokens.  The "tag" object at its
 simplest simply symbolically identifies another object by containing
 the sha1, type and symbolic name.
 
 However it can optionally contain additional signature information
 (which git doesn't care about as long as there's less than 8k of
 it). This can then be verified externally to git.
 
 Note that despite the tag features, "git" itself only handles content
 integrity; the trust framework (and signature provision and
 verification) has to come from outside.
 
 A tag is created with link:git-mktag.html[git-mktag] and
 -it's data can be accessed by link:git-cat-file.html[git-cat-file]
 +its data can be accessed by link:git-cat-file.html[git-cat-file]
 
 
 The "index" aka "Current Directory Cache"
 -----------------------------------------
 The index is a simple binary file, which contains an efficient
 representation of a virtual directory content at some random time.  It
 does so by a simple array that associates a set of names, dates,
 permissions and content (aka "blob") objects together.  The cache is
 always kept ordered by name, and names are unique (with a few very
 specific rules) at any point in time, but the cache has no long-term
 meaning, and can be partially updated at any time.
 
 In particular, the index certainly does not need to be consistent with
 the current directory contents (in fact, most operations will depend on
 different ways to make the index _not_ be consistent with the directory
 hierarchy), but it has three very important attributes:
 
 '(a) it can re-generate the full state it caches (not just the
 directory structure: it contains pointers to the "blob" objects so
 that it can regenerate the data too)'
 
 As a special case, there is a clear and unambiguous one-way mapping
 from a current directory cache to a "tree object", which can be
 efficiently created from just the current directory cache without
 actually looking at any other data.  So a directory cache at any one
 time uniquely specifies one and only one "tree" object (but has
 additional data to make it easy to match up that tree object with what
 has happened in the directory)
 
 '(b) it has efficient methods for finding inconsistencies between that
 cached state ("tree object waiting to be instantiated") and the
 current state.'
 
 '(c) it can additionally efficiently represent information about merge
 conflicts between different tree objects, allowing each pathname to be
 associated with sufficient information about the trees involved that
 you can create a three-way merge between them.'
 
 Those are the three ONLY things that the directory cache does.  It's a
 cache, and the normal operation is to re-generate it completely from a
 known tree object, or update/compare it with a live tree that is being
 developed.  If you blow the directory cache away entirely, you generally
 haven't lost any information as long as you have the name of the tree
 that it described. 
 
 At the same time, the directory index is at the same time also the
 staging area for creating new trees, and creating a new tree always
 involves a controlled modification of the index file.  In particular,
 the index file can have the representation of an intermediate tree that
 has not yet been instantiated.  So the index can be thought of as a
 write-back cache, which can contain dirty information that has not yet
 been written back to the backing store.
 
 
 
 The Workflow
 ------------
 Generally, all "git" operations work on the index file. Some operations
 work *purely* on the index file (showing the current state of the
 index), but most operations move data to and from the index file. Either
 from the database or from the working directory. Thus there are four
 main combinations: 
 
 1) working directory -> index
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You update the index with information from the working directory with
 the link:git-update-cache.html[git-update-cache] command.  You
 generally update the index information by just specifying the filename
 you want to update, like so:
 
 	git-update-cache filename
 
 but to avoid common mistakes with filename globbing etc, the command
 will not normally add totally new entries or remove old entries,
 i.e. it will normally just update existing cache entries.
 
 To tell git that yes, you really do realize that certain files no
 longer exist in the archive, or that new files should be added, you
 should use the "--remove" and "--add" flags respectively.
 
 NOTE! A "--remove" flag does _not_ mean that subsequent filenames will
 necessarily be removed: if the files still exist in your directory
 structure, the index will be updated with their new status, not
 removed. The only thing "--remove" means is that update-cache will be
 considering a removed file to be a valid thing, and if the file really
 does not exist any more, it will update the index accordingly.
 
 As a special case, you can also do "git-update-cache --refresh", which
 will refresh the "stat" information of each index to match the current
 stat information. It will _not_ update the object status itself, and
 it will only update the fields that are used to quickly test whether
 an object still matches its old backing store object.
 
 2) index -> object database
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You write your current index file to a "tree" object with the program
 
 	git-write-tree
 
 that doesn't come with any options - it will just write out the
 current index into the set of tree objects that describe that state,
 and it will return the name of the resulting top-level tree. You can
 use that tree to re-generate the index at any time by going in the
 other direction:
 
 3) object database -> index
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You read a "tree" file from the object database, and use that to
 populate (and overwrite - don't do this if your index contains any
 unsaved state that you might want to restore later!) your current
 index.  Normal operation is just
 
 		git-read-tree <sha1 of tree>
 
 and your index file will now be equivalent to the tree that you saved
 earlier. However, that is only your _index_ file: your working
 directory contents have not been modified.
 
 4) index -> working directory
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You update your working directory from the index by "checking out"
 files. This is not a very common operation, since normally you'd just
 keep your files updated, and rather than write to your working
 directory, you'd tell the index files about the changes in your
 working directory (i.e. "git-update-cache").
 
 However, if you decide to jump to a new version, or check out somebody
 else's version, or just restore a previous tree, you'd populate your
 index file with read-tree, and then you need to check out the result
 with
 		git-checkout-cache filename
 
 or, if you want to check out all of the index, use "-a".
 
 NOTE! git-checkout-cache normally refuses to overwrite old files, so
 if you have an old version of the tree already checked out, you will
 need to use the "-f" flag (_before_ the "-a" flag or the filename) to
 _force_ the checkout.
 
 
 Finally, there are a few odds and ends which are not purely moving
 from one representation to the other:
 
 5) Tying it all together
 ~~~~~~~~~~~~~~~~~~~~~~~~
 To commit a tree you have instantiated with "git-write-tree", you'd
 create a "commit" object that refers to that tree and the history
 behind it - most notably the "parent" commits that preceded it in
 history.
 
 Normally a "commit" has one parent: the previous state of the tree
 before a certain change was made. However, sometimes it can have two
 or more parent commits, in which case we call it a "merge", due to the
 fact that such a commit brings together ("merges") two or more
 previous states represented by other commits.
 
 In other words, while a "tree" represents a particular directory state
 of a working directory, a "commit" represents that state in "time",
 and explains how we got there.
 
 You create a commit object by giving it the tree that describes the
 state at the time of the commit, and a list of parents:
 
 	git-commit-tree <tree> -p <parent> [-p <parent2> ..]
 
 and then giving the reason for the commit on stdin (either through
 redirection from a pipe or file, or by just typing it at the tty).
 
 git-commit-tree will return the name of the object that represents
 that commit, and you should save it away for later use. Normally,
 you'd commit a new "HEAD" state, and while git doesn't care where you
 save the note about that state, in practice we tend to just write the
 result to the file ".git/HEAD", so that we can always see what the
 last committed state was.
 
 6) Examining the data
 ~~~~~~~~~~~~~~~~~~~~~
 
 You can examine the data represented in the object database and the
 index with various helper tools. For every object, you can use
 link:git-cat-file.html[git-cat-file] to examine details about the
 object:
 
 		git-cat-file -t <objectname>
 
 shows the type of the object, and once you have the type (which is
 usually implicit in where you find the object), you can use
 
 		git-cat-file blob|tree|commit <objectname>
 
 to show its contents. NOTE! Trees have binary content, and as a result
 there is a special helper for showing that content, called
 "git-ls-tree", which turns the binary content into a more easily
 readable form.
 
 It's especially instructive to look at "commit" objects, since those
 tend to be small and fairly self-explanatory. In particular, if you
 follow the convention of having the top commit name in ".git/HEAD",
 you can do
 
 		git-cat-file commit $(cat .git/HEAD)
 
 to see what the top commit was.
 
 7) Merging multiple trees
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Git helps you do a three-way merge, which you can expand to n-way by
 repeating the merge procedure arbitrary times until you finally
 "commit" the state.  The normal situation is that you'd only do one
 three-way merge (two parents), and commit it, but if you like to, you
 can do multiple parents in one go.
 
 To do a three-way merge, you need the two sets of "commit" objects
 that you want to merge, use those to find the closest common parent (a
 third "commit" object), and then use those commit objects to find the
 state of the directory ("tree" object) at these points.
 
 To get the "base" for the merge, you first look up the common parent
 of two commits with
 
 		git-merge-base <commit1> <commit2>
 
 which will return you the commit they are both based on.  You should
 now look up the "tree" objects of those commits, which you can easily
 do with (for example)
 
 		git-cat-file commit <commitname> | head -1
 
 since the tree object information is always the first line in a commit
 object.
 
 Once you know the three trees you are going to merge (the one
 "original" tree, aka the common case, and the two "result" trees, aka
 the branches you want to merge), you do a "merge" read into the
 index. This will throw away your old index contents, so you should
 make sure that you've committed those - in fact you would normally
 always do a merge against your last commit (which should thus match
 what you have in your current index anyway).
 
 To do the merge, do
 
 		git-read-tree -m <origtree> <target1tree> <target2tree>
 
 which will do all trivial merge operations for you directly in the
 index file, and you can just write the result out with
 "git-write-tree".
 
 NOTE! Because the merge is done in the index file, and not in your
 working directory, your working directory will no longer match your
 index. You can use "git-checkout-cache -f -a" to make the effect of
 the merge be seen in your working directory.
 
 NOTE2! Sadly, many merges aren't trivial. If there are files that have
 been added.moved or removed, or if both branches have modified the
 same file, you will be left with an index tree that contains "merge
 entries" in it. Such an index tree can _NOT_ be written out to a tree
 object, and you will have to resolve any such merge clashes using
 other tools before you can write out the result.
 
 
 [ fixme: talk about resolving merges here ]

 commit 7672db20c2060f20b01788e4a4289ebc5f818605
 Author: Bryan Larsen <[email protected]>
 Date:   Fri Jul 8 16:51:55 2005 -0700

    [PATCH] Expose object ID computation functions.
    
    This patch makes the first half of write_sha1_file() and
    index_fd() externally visible, to allow callers to compute the
    object ID without actually storing it in the object database.
    
    [JC demangled the whitespaces himself because he liked the patch
     so much, and reworked the interface to index_fd() slightly,
     taking suggestion from Linus and of his own.]
    
    Signed-off-by: Bryan Larsen <[email protected]>
    Signed-off-by: Junio C Hamano <[email protected]>
    Signed-off-by: Linus Torvalds <[email protected]>

 diff --git a/README b/README
 --- a/README
 +++ b/README
 @@ -17,466 +17,466 @@
 This is a stupid (but extremely fast) directory content manager.  It
 doesn't do a whole lot, but what it _does_ do is track directory
 contents efficiently. 
 
 There are two object abstractions: the "object database", and the
 "current directory cache" aka "index".
 
 The Object Database
 ~~~~~~~~~~~~~~~~~~~
 The object database is literally just a content-addressable collection
 of objects.  All objects are named by their content, which is
 approximated by the SHA1 hash of the object itself.  Objects may refer
 to other objects (by referencing their SHA1 hash), and so you can
 build up a hierarchy of objects.
 
 All objects have a statically determined "type" aka "tag", which is
 determined at object creation time, and which identifies the format of
 the object (i.e. how it is used, and how it can refer to other
 objects).  There are currently four different object types: "blob",
 "tree", "commit" and "tag".
 
 A "blob" object cannot refer to any other object, and is, like the tag
 implies, a pure storage object containing some user data.  It is used to
 actually store the file data, i.e. a blob object is associated with some
 particular version of some file. 
 
 A "tree" object is an object that ties one or more "blob" objects into a
 directory structure. In addition, a tree object can refer to other tree
 objects, thus creating a directory hierarchy. 
 
 A "commit" object ties such directory hierarchies together into
 a DAG of revisions - each "commit" is associated with exactly one tree
 (the directory hierarchy at the time of the commit). In addition, a
 "commit" refers to one or more "parent" commit objects that describe the
 history of how we arrived at that directory hierarchy.
 
 As a special case, a commit object with no parents is called the "root"
 object, and is the point of an initial project commit.  Each project
 must have at least one root, and while you can tie several different
 root objects together into one project by creating a commit object which
 has two or more separate roots as its ultimate parents, that's probably
 just going to confuse people.  So aim for the notion of "one root object
 per project", even if git itself does not enforce that. 
 
 A "tag" object symbolically identifies and can be used to sign other
 objects. It contains the identifier and type of another object, a
 symbolic name (of course!) and, optionally, a signature.
 
 Regardless of object type, all objects share the following
 characteristics: they are all deflated with zlib, and have a header
 that not only specifies their tag, but also provides size information
 about the data in the object.  It's worth noting that the SHA1 hash
 that is used to name the object is the hash of the original data.
 (Historical note: in the dawn of the age of git the hash
 was the sha1 of the _compressed_ object)
 
 As a result, the general consistency of an object can always be tested
 independently of the contents or the type of the object: all objects can
 be validated by verifying that (a) their hashes match the content of the
 file and (b) the object successfully inflates to a stream of bytes that
 forms a sequence of <ascii tag without space> + <space> + <ascii decimal
 size> + <byte\0> + <binary object data>. 
 
 The structured objects can further have their structure and
 connectivity to other objects verified. This is generally done with
 the "git-fsck-cache" program, which generates a full dependency graph
 of all objects, and verifies their internal consistency (in addition
 to just verifying their superficial consistency through the hash).
 
 The object types in some more detail:
 
 Blob Object
 ~~~~~~~~~~~
 A "blob" object is nothing but a binary blob of data, and doesn't
 refer to anything else.  There is no signature or any other
 verification of the data, so while the object is consistent (it _is_
 indexed by its sha1 hash, so the data itself is certainly correct), it
 has absolutely no other attributes.  No name associations, no
 permissions.  It is purely a blob of data (i.e. normally "file
 contents").
 
 In particular, since the blob is entirely defined by its data, if two
 files in a directory tree (or in multiple different versions of the
 repository) have the same contents, they will share the same blob
 object. The object is totally independent of it's location in the
 directory tree, and renaming a file does not change the object that
 file is associated with in any way.
 
 -A blob is created with link:git-write-blob.html[git-write-blob] and
 -it's data can be accessed by link:git-cat-file.html[git-cat-file]
 +A blob is typically created when link:git-update-cache.html[git-update-cache]
 +is run, and it's data can be accessed by link:git-cat-file.html[git-cat-file].
 
 Tree Object
 ~~~~~~~~~~~
 The next hierarchical object type is the "tree" object.  A tree object
 is a list of mode/name/blob data, sorted by name.  Alternatively, the
 mode data may specify a directory mode, in which case instead of
 naming a blob, that name is associated with another TREE object.
 
 Like the "blob" object, a tree object is uniquely determined by the
 set contents, and so two separate but identical trees will always
 share the exact same object. This is true at all levels, i.e. it's
 true for a "leaf" tree (which does not refer to any other trees, only
 blobs) as well as for a whole subdirectory.
 
 For that reason a "tree" object is just a pure data abstraction: it
 has no history, no signatures, no verification of validity, except
 that since the contents are again protected by the hash itself, we can
 trust that the tree is immutable and its contents never change.
 
 So you can trust the contents of a tree to be valid, the same way you
 can trust the contents of a blob, but you don't know where those
 contents _came_ from.
 
 Side note on trees: since a "tree" object is a sorted list of
 "filename+content", you can create a diff between two trees without
 actually having to unpack two trees.  Just ignore all common parts,
 and your diff will look right.  In other words, you can effectively
 (and efficiently) tell the difference between any two random trees by
 O(n) where "n" is the size of the difference, rather than the size of
 the tree.
 
 Side note 2 on trees: since the name of a "blob" depends entirely and
 exclusively on its contents (i.e. there are no names or permissions
 involved), you can see trivial renames or permission changes by
 noticing that the blob stayed the same.  However, renames with data
 changes need a smarter "diff" implementation.
 
 A tree is created with link:git-write-tree.html[git-write-tree] and
 it's data can be accessed by link:git-ls-tree.html[git-ls-tree]
 
 Commit Object
 ~~~~~~~~~~~~~
 The "commit" object is an object that introduces the notion of
 history into the picture.  In contrast to the other objects, it
 doesn't just describe the physical state of a tree, it describes how
 we got there, and why.
 
 A "commit" is defined by the tree-object that it results in, the
 parent commits (zero, one or more) that led up to that point, and a
 comment on what happened.  Again, a commit is not trusted per se:
 the contents are well-defined and "safe" due to the cryptographically
 strong signatures at all levels, but there is no reason to believe
 that the tree is "good" or that the merge information makes sense.
 The parents do not have to actually have any relationship with the
 result, for example.
 
 Note on commits: unlike real SCM's, commits do not contain
 rename information or file mode chane information.  All of that is
 implicit in the trees involved (the result tree, and the result trees
 of the parents), and describing that makes no sense in this idiotic
 file manager.
 
 A commit is created with link:git-commit-tree.html[git-commit-tree] and
 it's data can be accessed by link:git-cat-file.html[git-cat-file]
 
 Trust
 ~~~~~
 An aside on the notion of "trust". Trust is really outside the scope
 of "git", but it's worth noting a few things.  First off, since
 everything is hashed with SHA1, you _can_ trust that an object is
 intact and has not been messed with by external sources.  So the name
 of an object uniquely identifies a known state - just not a state that
 you may want to trust.
 
 Furthermore, since the SHA1 signature of a commit refers to the
 SHA1 signatures of the tree it is associated with and the signatures
 of the parent, a single named commit specifies uniquely a whole set
 of history, with full contents.  You can't later fake any step of the
 way once you have the name of a commit.
 
 So to introduce some real trust in the system, the only thing you need
 to do is to digitally sign just _one_ special note, which includes the
 name of a top-level commit.  Your digital signature shows others
 that you trust that commit, and the immutability of the history of
 commits tells others that they can trust the whole history.
 
 In other words, you can easily validate a whole archive by just
 sending out a single email that tells the people the name (SHA1 hash)
 of the top commit, and digitally sign that email using something
 like GPG/PGP.
 
 To assist in this, git also provides the tag object...
 
 Tag Object
 ~~~~~~~~~~
 Git provides the "tag" object to simplify creating, managing and
 exchanging symbolic and signed tokens.  The "tag" object at its
 simplest simply symbolically identifies another object by containing
 the sha1, type and symbolic name.
 
 However it can optionally contain additional signature information
 (which git doesn't care about as long as there's less than 8k of
 it). This can then be verified externally to git.
 
 Note that despite the tag features, "git" itself only handles content
 integrity; the trust framework (and signature provision and
 verification) has to come from outside.
 
 A tag is created with link:git-mktag.html[git-mktag] and
 it's data can be accessed by link:git-cat-file.html[git-cat-file]
 
 
 The "index" aka "Current Directory Cache"
 -----------------------------------------
 The index is a simple binary file, which contains an efficient
 representation of a virtual directory content at some random time.  It
 does so by a simple array that associates a set of names, dates,
 permissions and content (aka "blob") objects together.  The cache is
 always kept ordered by name, and names are unique (with a few very
 specific rules) at any point in time, but the cache has no long-term
 meaning, and can be partially updated at any time.
 
 In particular, the index certainly does not need to be consistent with
 the current directory contents (in fact, most operations will depend on
 different ways to make the index _not_ be consistent with the directory
 hierarchy), but it has three very important attributes:
 
 '(a) it can re-generate the full state it caches (not just the
 directory structure: it contains pointers to the "blob" objects so
 that it can regenerate the data too)'
 
 As a special case, there is a clear and unambiguous one-way mapping
 from a current directory cache to a "tree object", which can be
 efficiently created from just the current directory cache without
 actually looking at any other data.  So a directory cache at any one
 time uniquely specifies one and only one "tree" object (but has
 additional data to make it easy to match up that tree object with what
 has happened in the directory)
 
 '(b) it has efficient methods for finding inconsistencies between that
 cached state ("tree object waiting to be instantiated") and the
 current state.'
 
 '(c) it can additionally efficiently represent information about merge
 conflicts between different tree objects, allowing each pathname to be
 associated with sufficient information about the trees involved that
 you can create a three-way merge between them.'
 
 Those are the three ONLY things that the directory cache does.  It's a
 cache, and the normal operation is to re-generate it completely from a
 known tree object, or update/compare it with a live tree that is being
 developed.  If you blow the directory cache away entirely, you generally
 haven't lost any information as long as you have the name of the tree
 that it described. 
 
 At the same time, the directory index is at the same time also the
 staging area for creating new trees, and creating a new tree always
 involves a controlled modification of the index file.  In particular,
 the index file can have the representation of an intermediate tree that
 has not yet been instantiated.  So the index can be thought of as a
 write-back cache, which can contain dirty information that has not yet
 been written back to the backing store.
 
 
 
 The Workflow
 ------------
 Generally, all "git" operations work on the index file. Some operations
 work *purely* on the index file (showing the current state of the
 index), but most operations move data to and from the index file. Either
 from the database or from the working directory. Thus there are four
 main combinations: 
 
 1) working directory -> index
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You update the index with information from the working directory with
 the link:git-update-cache.html[git-update-cache] command.  You
 generally update the index information by just specifying the filename
 you want to update, like so:
 
 	git-update-cache filename
 
 but to avoid common mistakes with filename globbing etc, the command
 will not normally add totally new entries or remove old entries,
 i.e. it will normally just update existing cache entries.
 
 To tell git that yes, you really do realize that certain files no
 longer exist in the archive, or that new files should be added, you
 should use the "--remove" and "--add" flags respectively.
 
 NOTE! A "--remove" flag does _not_ mean that subsequent filenames will
 necessarily be removed: if the files still exist in your directory
 structure, the index will be updated with their new status, not
 removed. The only thing "--remove" means is that update-cache will be
 considering a removed file to be a valid thing, and if the file really
 does not exist any more, it will update the index accordingly.
 
 As a special case, you can also do "git-update-cache --refresh", which
 will refresh the "stat" information of each index to match the current
 stat information. It will _not_ update the object status itself, and
 it will only update the fields that are used to quickly test whether
 an object still matches its old backing store object.
 
 2) index -> object database
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You write your current index file to a "tree" object with the program
 
 	git-write-tree
 
 that doesn't come with any options - it will just write out the
 current index into the set of tree objects that describe that state,
 and it will return the name of the resulting top-level tree. You can
 use that tree to re-generate the index at any time by going in the
 other direction:
 
 3) object database -> index
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You read a "tree" file from the object database, and use that to
 populate (and overwrite - don't do this if your index contains any
 unsaved state that you might want to restore later!) your current
 index.  Normal operation is just
 
 		git-read-tree <sha1 of tree>
 
 and your index file will now be equivalent to the tree that you saved
 earlier. However, that is only your _index_ file: your working
 directory contents have not been modified.
 
 4) index -> working directory
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You update your working directory from the index by "checking out"
 files. This is not a very common operation, since normally you'd just
 keep your files updated, and rather than write to your working
 directory, you'd tell the index files about the changes in your
 working directory (i.e. "git-update-cache").
 
 However, if you decide to jump to a new version, or check out somebody
 else's version, or just restore a previous tree, you'd populate your
 index file with read-tree, and then you need to check out the result
 with
 		git-checkout-cache filename
 
 or, if you want to check out all of the index, use "-a".
 
 NOTE! git-checkout-cache normally refuses to overwrite old files, so
 if you have an old version of the tree already checked out, you will
 need to use the "-f" flag (_before_ the "-a" flag or the filename) to
 _force_ the checkout.
 
 
 Finally, there are a few odds and ends which are not purely moving
 from one representation to the other:
 
 5) Tying it all together
 ~~~~~~~~~~~~~~~~~~~~~~~~
 To commit a tree you have instantiated with "git-write-tree", you'd
 create a "commit" object that refers to that tree and the history
 behind it - most notably the "parent" commits that preceded it in
 history.
 
 Normally a "commit" has one parent: the previous state of the tree
 before a certain change was made. However, sometimes it can have two
 or more parent commits, in which case we call it a "merge", due to the
 fact that such a commit brings together ("merges") two or more
 previous states represented by other commits.
 
 In other words, while a "tree" represents a particular directory state
 of a working directory, a "commit" represents that state in "time",
 and explains how we got there.
 
 You create a commit object by giving it the tree that describes the
 state at the time of the commit, and a list of parents:
 
 	git-commit-tree <tree> -p <parent> [-p <parent2> ..]
 
 and then giving the reason for the commit on stdin (either through
 redirection from a pipe or file, or by just typing it at the tty).
 
 git-commit-tree will return the name of the object that represents
 that commit, and you should save it away for later use. Normally,
 you'd commit a new "HEAD" state, and while git doesn't care where you
 save the note about that state, in practice we tend to just write the
 result to the file ".git/HEAD", so that we can always see what the
 last committed state was.
 
 6) Examining the data
 ~~~~~~~~~~~~~~~~~~~~~
 
 You can examine the data represented in the object database and the
 index with various helper tools. For every object, you can use
 link:git-cat-file.html[git-cat-file] to examine details about the
 object:
 
 		git-cat-file -t <objectname>
 
 shows the type of the object, and once you have the type (which is
 usually implicit in where you find the object), you can use
 
 		git-cat-file blob|tree|commit <objectname>
 
 to show its contents. NOTE! Trees have binary content, and as a result
 there is a special helper for showing that content, called
 "git-ls-tree", which turns the binary content into a more easily
 readable form.
 
 It's especially instructive to look at "commit" objects, since those
 tend to be small and fairly self-explanatory. In particular, if you
 follow the convention of having the top commit name in ".git/HEAD",
 you can do
 
 		git-cat-file commit $(cat .git/HEAD)
 
 to see what the top commit was.
 
 7) Merging multiple trees
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Git helps you do a three-way merge, which you can expand to n-way by
 repeating the merge procedure arbitrary times until you finally
 "commit" the state.  The normal situation is that you'd only do one
 three-way merge (two parents), and commit it, but if you like to, you
 can do multiple parents in one go.
 
 To do a three-way merge, you need the two sets of "commit" objects
 that you want to merge, use those to find the closest common parent (a
 third "commit" object), and then use those commit objects to find the
 state of the directory ("tree" object) at these points.
 
 To get the "base" for the merge, you first look up the common parent
 of two commits with
 
 		git-merge-base <commit1> <commit2>
 
 which will return you the commit they are both based on.  You should
 now look up the "tree" objects of those commits, which you can easily
 do with (for example)
 
 		git-cat-file commit <commitname> | head -1
 
 since the tree object information is always the first line in a commit
 object.
 
 Once you know the three trees you are going to merge (the one
 "original" tree, aka the common case, and the two "result" trees, aka
 the branches you want to merge), you do a "merge" read into the
 index. This will throw away your old index contents, so you should
 make sure that you've committed those - in fact you would normally
 always do a merge against your last commit (which should thus match
 what you have in your current index anyway).
 
 To do the merge, do
 
 		git-read-tree -m <origtree> <target1tree> <target2tree>
 
 which will do all trivial merge operations for you directly in the
 index file, and you can just write the result out with
 "git-write-tree".
 
 NOTE! Because the merge is done in the index file, and not in your
 working directory, your working directory will no longer match your
 index. You can use "git-checkout-cache -f -a" to make the effect of
 the merge be seen in your working directory.
 
 NOTE2! Sadly, many merges aren't trivial. If there are files that have
 been added.moved or removed, or if both branches have modified the
 same file, you will be left with an index tree that contains "merge
 entries" in it. Such an index tree can _NOT_ be written out to a tree
 object, and you will have to resolve any such merge clashes using
 other tools before you can write out the result.
 
 
 [ fixme: talk about resolving merges here ]

 commit c4584ae3fd7cd595a638a07dfd853e9d2745e930
 Author: Junio C Hamano <[email protected]>
 Date:   Mon Jun 27 03:33:33 2005 -0700

    [PATCH] Remove "delta" object representation.
    
    Packed delta files created by git-pack-objects seems to be the
    way to go, and existing "delta" object handling code has exposed
    the object representation details to too many places.  Remove it
    while we refactor code to come up with a proper interface in
    sha1_file.c.
    
    Signed-off-by: Junio C Hamano <[email protected]>
    Signed-off-by: Linus Torvalds <[email protected]>

 diff --git a/README b/README
 --- a/README
 +++ b/README
 @@ -17,493 +17,466 @@
 This is a stupid (but extremely fast) directory content manager.  It
 doesn't do a whole lot, but what it _does_ do is track directory
 contents efficiently. 
 
 There are two object abstractions: the "object database", and the
 "current directory cache" aka "index".
 
 The Object Database
 ~~~~~~~~~~~~~~~~~~~
 The object database is literally just a content-addressable collection
 of objects.  All objects are named by their content, which is
 approximated by the SHA1 hash of the object itself.  Objects may refer
 to other objects (by referencing their SHA1 hash), and so you can
 build up a hierarchy of objects.
 
 All objects have a statically determined "type" aka "tag", which is
 determined at object creation time, and which identifies the format of
 the object (i.e. how it is used, and how it can refer to other
 -objects).  There are currently five different object types: "blob",
 -"tree", "commit", "tag" and "delta"
 +objects).  There are currently four different object types: "blob",
 +"tree", "commit" and "tag".
 
 A "blob" object cannot refer to any other object, and is, like the tag
 implies, a pure storage object containing some user data.  It is used to
 actually store the file data, i.e. a blob object is associated with some
 particular version of some file. 
 
 A "tree" object is an object that ties one or more "blob" objects into a
 directory structure. In addition, a tree object can refer to other tree
 objects, thus creating a directory hierarchy. 
 
 A "commit" object ties such directory hierarchies together into
 a DAG of revisions - each "commit" is associated with exactly one tree
 (the directory hierarchy at the time of the commit). In addition, a
 "commit" refers to one or more "parent" commit objects that describe the
 history of how we arrived at that directory hierarchy.
 
 As a special case, a commit object with no parents is called the "root"
 object, and is the point of an initial project commit.  Each project
 must have at least one root, and while you can tie several different
 root objects together into one project by creating a commit object which
 has two or more separate roots as its ultimate parents, that's probably
 just going to confuse people.  So aim for the notion of "one root object
 per project", even if git itself does not enforce that. 
 
 A "tag" object symbolically identifies and can be used to sign other
 objects. It contains the identifier and type of another object, a
 symbolic name (of course!) and, optionally, a signature.
 
 -A "delta" object is used internally by the object database to minimise
 -disk usage. Instead of storing the entire contents of a revision, git
 -can behave in a similar manner to RCS et al and simply store a delta.
 -
 Regardless of object type, all objects share the following
 characteristics: they are all deflated with zlib, and have a header
 that not only specifies their tag, but also provides size information
 about the data in the object.  It's worth noting that the SHA1 hash
 -that is used to name the object is the hash of the original data or
 -the delta. (Historical note: in the dawn of the age of git the hash
 +that is used to name the object is the hash of the original data.
 +(Historical note: in the dawn of the age of git the hash
 was the sha1 of the _compressed_ object)
 
 As a result, the general consistency of an object can always be tested
 independently of the contents or the type of the object: all objects can
 be validated by verifying that (a) their hashes match the content of the
 file and (b) the object successfully inflates to a stream of bytes that
 forms a sequence of <ascii tag without space> + <space> + <ascii decimal
 size> + <byte\0> + <binary object data>. 
 
 The structured objects can further have their structure and
 connectivity to other objects verified. This is generally done with
 the "git-fsck-cache" program, which generates a full dependency graph
 of all objects, and verifies their internal consistency (in addition
 to just verifying their superficial consistency through the hash).
 
 The object types in some more detail:
 
 Blob Object
 ~~~~~~~~~~~
 A "blob" object is nothing but a binary blob of data, and doesn't
 refer to anything else.  There is no signature or any other
 verification of the data, so while the object is consistent (it _is_
 indexed by its sha1 hash, so the data itself is certainly correct), it
 has absolutely no other attributes.  No name associations, no
 permissions.  It is purely a blob of data (i.e. normally "file
 contents").
 
 In particular, since the blob is entirely defined by its data, if two
 files in a directory tree (or in multiple different versions of the
 repository) have the same contents, they will share the same blob
 object. The object is totally independent of it's location in the
 directory tree, and renaming a file does not change the object that
 file is associated with in any way.
 
 A blob is created with link:git-write-blob.html[git-write-blob] and
 it's data can be accessed by link:git-cat-file.html[git-cat-file]
 
 Tree Object
 ~~~~~~~~~~~
 The next hierarchical object type is the "tree" object.  A tree object
 is a list of mode/name/blob data, sorted by name.  Alternatively, the
 mode data may specify a directory mode, in which case instead of
 naming a blob, that name is associated with another TREE object.
 
 Like the "blob" object, a tree object is uniquely determined by the
 set contents, and so two separate but identical trees will always
 share the exact same object. This is true at all levels, i.e. it's
 true for a "leaf" tree (which does not refer to any other trees, only
 blobs) as well as for a whole subdirectory.
 
 For that reason a "tree" object is just a pure data abstraction: it
 has no history, no signatures, no verification of validity, except
 that since the contents are again protected by the hash itself, we can
 trust that the tree is immutable and its contents never change.
 
 So you can trust the contents of a tree to be valid, the same way you
 can trust the contents of a blob, but you don't know where those
 contents _came_ from.
 
 Side note on trees: since a "tree" object is a sorted list of
 "filename+content", you can create a diff between two trees without
 actually having to unpack two trees.  Just ignore all common parts,
 and your diff will look right.  In other words, you can effectively
 (and efficiently) tell the difference between any two random trees by
 O(n) where "n" is the size of the difference, rather than the size of
 the tree.
 
 Side note 2 on trees: since the name of a "blob" depends entirely and
 exclusively on its contents (i.e. there are no names or permissions
 involved), you can see trivial renames or permission changes by
 noticing that the blob stayed the same.  However, renames with data
 changes need a smarter "diff" implementation.
 
 A tree is created with link:git-write-tree.html[git-write-tree] and
 it's data can be accessed by link:git-ls-tree.html[git-ls-tree]
 
 Commit Object
 ~~~~~~~~~~~~~
 The "commit" object is an object that introduces the notion of
 history into the picture.  In contrast to the other objects, it
 doesn't just describe the physical state of a tree, it describes how
 we got there, and why.
 
 A "commit" is defined by the tree-object that it results in, the
 parent commits (zero, one or more) that led up to that point, and a
 comment on what happened.  Again, a commit is not trusted per se:
 the contents are well-defined and "safe" due to the cryptographically
 strong signatures at all levels, but there is no reason to believe
 that the tree is "good" or that the merge information makes sense.
 The parents do not have to actually have any relationship with the
 result, for example.
 
 Note on commits: unlike real SCM's, commits do not contain
 rename information or file mode chane information.  All of that is
 implicit in the trees involved (the result tree, and the result trees
 of the parents), and describing that makes no sense in this idiotic
 file manager.
 
 A commit is created with link:git-commit-tree.html[git-commit-tree] and
 it's data can be accessed by link:git-cat-file.html[git-cat-file]
 
 Trust
 ~~~~~
 An aside on the notion of "trust". Trust is really outside the scope
 of "git", but it's worth noting a few things.  First off, since
 everything is hashed with SHA1, you _can_ trust that an object is
 intact and has not been messed with by external sources.  So the name
 of an object uniquely identifies a known state - just not a state that
 you may want to trust.
 
 Furthermore, since the SHA1 signature of a commit refers to the
 SHA1 signatures of the tree it is associated with and the signatures
 of the parent, a single named commit specifies uniquely a whole set
 of history, with full contents.  You can't later fake any step of the
 way once you have the name of a commit.
 
 So to introduce some real trust in the system, the only thing you need
 to do is to digitally sign just _one_ special note, which includes the
 name of a top-level commit.  Your digital signature shows others
 that you trust that commit, and the immutability of the history of
 commits tells others that they can trust the whole history.
 
 In other words, you can easily validate a whole archive by just
 sending out a single email that tells the people the name (SHA1 hash)
 of the top commit, and digitally sign that email using something
 like GPG/PGP.
 
 To assist in this, git also provides the tag object...
 
 Tag Object
 ~~~~~~~~~~
 Git provides the "tag" object to simplify creating, managing and
 exchanging symbolic and signed tokens.  The "tag" object at its
 simplest simply symbolically identifies another object by containing
 the sha1, type and symbolic name.
 
 However it can optionally contain additional signature information
 (which git doesn't care about as long as there's less than 8k of
 it). This can then be verified externally to git.
 
 Note that despite the tag features, "git" itself only handles content
 integrity; the trust framework (and signature provision and
 verification) has to come from outside.
 
 A tag is created with link:git-mktag.html[git-mktag] and
 it's data can be accessed by link:git-cat-file.html[git-cat-file]
 
 -Delta Object
 -~~~~~~~~~~~~
 -
 -The "delta" object is used internally by the object database to
 -minimise storage usage by using xdeltas (byte level diffs). Deltas can
 -form chains of arbitrary length as RCS does (although this is
 -configureable at creation time). Most operations won't see or even be
 -aware of delta objects as they are automatically 'applied' and appear
 -as 'real' git objects In other words, if you write your own routines
 -to look at the contents of the object database then you need to know
 -about this - otherwise you don't. Actually, that's not quite true -
 -one important area where deltas are likely to prove very valuable is
 -in reducing bandwidth loads - so the more sophisticated network tools
 -for git repositories will be aware of them too.
 -
 -Finally, git repositories can (and must) be deltafied in the
 -background - the work to calculate the differences does not take place
 -automatically at commit time.
 -
 -A delta can be created (or undeltafied) with
 -link:git-mkdelta.html[git-mkdelta] it's raw data cannot be accessed at
 -present.
 -
 
 The "index" aka "Current Directory Cache"
 -----------------------------------------
 The index is a simple binary file, which contains an efficient
 representation of a virtual directory content at some random time.  It
 does so by a simple array that associates a set of names, dates,
 permissions and content (aka "blob") objects together.  The cache is
 always kept ordered by name, and names are unique (with a few very
 specific rules) at any point in time, but the cache has no long-term
 meaning, and can be partially updated at any time.
 
 In particular, the index certainly does not need to be consistent with
 the current directory contents (in fact, most operations will depend on
 different ways to make the index _not_ be consistent with the directory
 hierarchy), but it has three very important attributes:
 
 '(a) it can re-generate the full state it caches (not just the
 directory structure: it contains pointers to the "blob" objects so
 that it can regenerate the data too)'
 
 As a special case, there is a clear and unambiguous one-way mapping
 from a current directory cache to a "tree object", which can be
 efficiently created from just the current directory cache without
 actually looking at any other data.  So a directory cache at any one
 time uniquely specifies one and only one "tree" object (but has
 additional data to make it easy to match up that tree object with what
 has happened in the directory)
 
 '(b) it has efficient methods for finding inconsistencies between that
 cached state ("tree object waiting to be instantiated") and the
 current state.'
 
 '(c) it can additionally efficiently represent information about merge
 conflicts between different tree objects, allowing each pathname to be
 associated with sufficient information about the trees involved that
 you can create a three-way merge between them.'
 
 Those are the three ONLY things that the directory cache does.  It's a
 cache, and the normal operation is to re-generate it completely from a
 known tree object, or update/compare it with a live tree that is being
 developed.  If you blow the directory cache away entirely, you generally
 haven't lost any information as long as you have the name of the tree
 that it described. 
 
 At the same time, the directory index is at the same time also the
 staging area for creating new trees, and creating a new tree always
 involves a controlled modification of the index file.  In particular,
 the index file can have the representation of an intermediate tree that
 has not yet been instantiated.  So the index can be thought of as a
 write-back cache, which can contain dirty information that has not yet
 been written back to the backing store.
 
 
 
 The Workflow
 ------------
 Generally, all "git" operations work on the index file. Some operations
 work *purely* on the index file (showing the current state of the
 index), but most operations move data to and from the index file. Either
 from the database or from the working directory. Thus there are four
 main combinations: 
 
 1) working directory -> index
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You update the index with information from the working directory with
 the link:git-update-cache.html[git-update-cache] command.  You
 generally update the index information by just specifying the filename
 you want to update, like so:
 
 	git-update-cache filename
 
 but to avoid common mistakes with filename globbing etc, the command
 will not normally add totally new entries or remove old entries,
 i.e. it will normally just update existing cache entries.
 
 To tell git that yes, you really do realize that certain files no
 longer exist in the archive, or that new files should be added, you
 should use the "--remove" and "--add" flags respectively.
 
 NOTE! A "--remove" flag does _not_ mean that subsequent filenames will
 necessarily be removed: if the files still exist in your directory
 structure, the index will be updated with their new status, not
 removed. The only thing "--remove" means is that update-cache will be
 considering a removed file to be a valid thing, and if the file really
 does not exist any more, it will update the index accordingly.
 
 As a special case, you can also do "git-update-cache --refresh", which
 will refresh the "stat" information of each index to match the current
 stat information. It will _not_ update the object status itself, and
 it will only update the fields that are used to quickly test whether
 an object still matches its old backing store object.
 
 2) index -> object database
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You write your current index file to a "tree" object with the program
 
 	git-write-tree
 
 that doesn't come with any options - it will just write out the
 current index into the set of tree objects that describe that state,
 and it will return the name of the resulting top-level tree. You can
 use that tree to re-generate the index at any time by going in the
 other direction:
 
 3) object database -> index
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You read a "tree" file from the object database, and use that to
 populate (and overwrite - don't do this if your index contains any
 unsaved state that you might want to restore later!) your current
 index.  Normal operation is just
 
 		git-read-tree <sha1 of tree>
 
 and your index file will now be equivalent to the tree that you saved
 earlier. However, that is only your _index_ file: your working
 directory contents have not been modified.
 
 4) index -> working directory
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You update your working directory from the index by "checking out"
 files. This is not a very common operation, since normally you'd just
 keep your files updated, and rather than write to your working
 directory, you'd tell the index files about the changes in your
 working directory (i.e. "git-update-cache").
 
 However, if you decide to jump to a new version, or check out somebody
 else's version, or just restore a previous tree, you'd populate your
 index file with read-tree, and then you need to check out the result
 with
 		git-checkout-cache filename
 
 or, if you want to check out all of the index, use "-a".
 
 NOTE! git-checkout-cache normally refuses to overwrite old files, so
 if you have an old version of the tree already checked out, you will
 need to use the "-f" flag (_before_ the "-a" flag or the filename) to
 _force_ the checkout.
 
 
 Finally, there are a few odds and ends which are not purely moving
 from one representation to the other:
 
 5) Tying it all together
 ~~~~~~~~~~~~~~~~~~~~~~~~
 To commit a tree you have instantiated with "git-write-tree", you'd
 create a "commit" object that refers to that tree and the history
 behind it - most notably the "parent" commits that preceded it in
 history.
 
 Normally a "commit" has one parent: the previous state of the tree
 before a certain change was made. However, sometimes it can have two
 or more parent commits, in which case we call it a "merge", due to the
 fact that such a commit brings together ("merges") two or more
 previous states represented by other commits.
 
 In other words, while a "tree" represents a particular directory state
 of a working directory, a "commit" represents that state in "time",
 and explains how we got there.
 
 You create a commit object by giving it the tree that describes the
 state at the time of the commit, and a list of parents:
 
 	git-commit-tree <tree> -p <parent> [-p <parent2> ..]
 
 and then giving the reason for the commit on stdin (either through
 redirection from a pipe or file, or by just typing it at the tty).
 
 git-commit-tree will return the name of the object that represents
 that commit, and you should save it away for later use. Normally,
 you'd commit a new "HEAD" state, and while git doesn't care where you
 save the note about that state, in practice we tend to just write the
 result to the file ".git/HEAD", so that we can always see what the
 last committed state was.
 
 6) Examining the data
 ~~~~~~~~~~~~~~~~~~~~~
 
 You can examine the data represented in the object database and the
 index with various helper tools. For every object, you can use
 link:git-cat-file.html[git-cat-file] to examine details about the
 object:
 
 		git-cat-file -t <objectname>
 
 shows the type of the object, and once you have the type (which is
 usually implicit in where you find the object), you can use
 
 		git-cat-file blob|tree|commit <objectname>
 
 to show its contents. NOTE! Trees have binary content, and as a result
 there is a special helper for showing that content, called
 "git-ls-tree", which turns the binary content into a more easily
 readable form.
 
 It's especially instructive to look at "commit" objects, since those
 tend to be small and fairly self-explanatory. In particular, if you
 follow the convention of having the top commit name in ".git/HEAD",
 you can do
 
 		git-cat-file commit $(cat .git/HEAD)
 
 to see what the top commit was.
 
 7) Merging multiple trees
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Git helps you do a three-way merge, which you can expand to n-way by
 repeating the merge procedure arbitrary times until you finally
 "commit" the state.  The normal situation is that you'd only do one
 three-way merge (two parents), and commit it, but if you like to, you
 can do multiple parents in one go.
 
 To do a three-way merge, you need the two sets of "commit" objects
 that you want to merge, use those to find the closest common parent (a
 third "commit" object), and then use those commit objects to find the
 state of the directory ("tree" object) at these points.
 
 To get the "base" for the merge, you first look up the common parent
 of two commits with
 
 		git-merge-base <commit1> <commit2>
 
 which will return you the commit they are both based on.  You should
 now look up the "tree" objects of those commits, which you can easily
 do with (for example)
 
 		git-cat-file commit <commitname> | head -1
 
 since the tree object information is always the first line in a commit
 object.
 
 Once you know the three trees you are going to merge (the one
 "original" tree, aka the common case, and the two "result" trees, aka
 the branches you want to merge), you do a "merge" read into the
 index. This will throw away your old index contents, so you should
 make sure that you've committed those - in fact you would normally
 always do a merge against your last commit (which should thus match
 what you have in your current index anyway).
 
 To do the merge, do
 
 		git-read-tree -m <origtree> <target1tree> <target2tree>
 
 which will do all trivial merge operations for you directly in the
 index file, and you can just write the result out with
 "git-write-tree".
 
 NOTE! Because the merge is done in the index file, and not in your
 working directory, your working directory will no longer match your
 index. You can use "git-checkout-cache -f -a" to make the effect of
 the merge be seen in your working directory.
 
 NOTE2! Sadly, many merges aren't trivial. If there are files that have
 been added.moved or removed, or if both branches have modified the
 same file, you will be left with an index tree that contains "merge
 entries" in it. Such an index tree can _NOT_ be written out to a tree
 object, and you will have to resolve any such merge clashes using
 other tools before you can write out the result.
 
 
 [ fixme: talk about resolving merges here ]

 commit 2aef5bbae99aeba3551408eae13faea02bf55b67
 Author: David Greaves <[email protected]>
 Date:   Sun May 22 18:44:17 2005 +0100

    [PATCH] Docs - delta object
    
    Added delta documentation
    
    Signed-off-by: David Greaves <[email protected]>
    Signed-off-by: Linus Torvalds <[email protected]>

 diff --git a/README b/README
 --- a/README
 +++ b/README
 @@ -17,465 +17,493 @@
 This is a stupid (but extremely fast) directory content manager.  It
 doesn't do a whole lot, but what it _does_ do is track directory
 contents efficiently. 
 
 There are two object abstractions: the "object database", and the
 "current directory cache" aka "index".
 
 The Object Database
 ~~~~~~~~~~~~~~~~~~~
 The object database is literally just a content-addressable collection
 of objects.  All objects are named by their content, which is
 approximated by the SHA1 hash of the object itself.  Objects may refer
 to other objects (by referencing their SHA1 hash), and so you can
 build up a hierarchy of objects.
 
 All objects have a statically determined "type" aka "tag", which is
 determined at object creation time, and which identifies the format of
 the object (i.e. how it is used, and how it can refer to other
 -objects).  There are currently four different object types: "blob",
 -"tree", "commit" and "tag".
 +objects).  There are currently five different object types: "blob",
 +"tree", "commit", "tag" and "delta"
 
 A "blob" object cannot refer to any other object, and is, like the tag
 implies, a pure storage object containing some user data.  It is used to
 actually store the file data, i.e. a blob object is associated with some
 particular version of some file. 
 
 A "tree" object is an object that ties one or more "blob" objects into a
 directory structure. In addition, a tree object can refer to other tree
 objects, thus creating a directory hierarchy. 
 
 A "commit" object ties such directory hierarchies together into
 a DAG of revisions - each "commit" is associated with exactly one tree
 (the directory hierarchy at the time of the commit). In addition, a
 "commit" refers to one or more "parent" commit objects that describe the
 history of how we arrived at that directory hierarchy.
 
 As a special case, a commit object with no parents is called the "root"
 object, and is the point of an initial project commit.  Each project
 must have at least one root, and while you can tie several different
 root objects together into one project by creating a commit object which
 has two or more separate roots as its ultimate parents, that's probably
 just going to confuse people.  So aim for the notion of "one root object
 per project", even if git itself does not enforce that. 
 
 A "tag" object symbolically identifies and can be used to sign other
 objects. It contains the identifier and type of another object, a
 symbolic name (of course!) and, optionally, a signature.
 
 -Regardless of object type, all objects are share the following
 -characteristics: they are all in deflated with zlib, and have a header
 -that not only specifies their tag, but also size information about the
 -data in the object.  It's worth noting that the SHA1 hash that is used
 -to name the object is the hash of the original data (historical note:
 -in the dawn of the age of git this was the sha1 of the _compressed_
 -object)
 +A "delta" object is used internally by the object database to minimise
 +disk usage. Instead of storing the entire contents of a revision, git
 +can behave in a similar manner to RCS et al and simply store a delta.
 +
 +Regardless of object type, all objects share the following
 +characteristics: they are all deflated with zlib, and have a header
 +that not only specifies their tag, but also provides size information
 +about the data in the object.  It's worth noting that the SHA1 hash
 +that is used to name the object is the hash of the original data or
 +the delta. (Historical note: in the dawn of the age of git the hash
 +was the sha1 of the _compressed_ object)
 
 As a result, the general consistency of an object can always be tested
 independently of the contents or the type of the object: all objects can
 be validated by verifying that (a) their hashes match the content of the
 file and (b) the object successfully inflates to a stream of bytes that
 forms a sequence of <ascii tag without space> + <space> + <ascii decimal
 size> + <byte\0> + <binary object data>. 
 
 The structured objects can further have their structure and
 connectivity to other objects verified. This is generally done with
 the "git-fsck-cache" program, which generates a full dependency graph
 of all objects, and verifies their internal consistency (in addition
 to just verifying their superficial consistency through the hash).
 
 The object types in some more detail:
 
 Blob Object
 ~~~~~~~~~~~
 A "blob" object is nothing but a binary blob of data, and doesn't
 refer to anything else.  There is no signature or any other
 verification of the data, so while the object is consistent (it _is_
 indexed by its sha1 hash, so the data itself is certainly correct), it
 has absolutely no other attributes.  No name associations, no
 permissions.  It is purely a blob of data (i.e. normally "file
 contents").
 
 In particular, since the blob is entirely defined by its data, if two
 files in a directory tree (or in multiple different versions of the
 repository) have the same contents, they will share the same blob
 object. The object is totally independent of it's location in the
 directory tree, and renaming a file does not change the object that
 file is associated with in any way.
 
 A blob is created with link:git-write-blob.html[git-write-blob] and
 it's data can be accessed by link:git-cat-file.html[git-cat-file]
 
 Tree Object
 ~~~~~~~~~~~
 The next hierarchical object type is the "tree" object.  A tree object
 is a list of mode/name/blob data, sorted by name.  Alternatively, the
 mode data may specify a directory mode, in which case instead of
 naming a blob, that name is associated with another TREE object.
 
 Like the "blob" object, a tree object is uniquely determined by the
 set contents, and so two separate but identical trees will always
 share the exact same object. This is true at all levels, i.e. it's
 true for a "leaf" tree (which does not refer to any other trees, only
 blobs) as well as for a whole subdirectory.
 
 For that reason a "tree" object is just a pure data abstraction: it
 has no history, no signatures, no verification of validity, except
 that since the contents are again protected by the hash itself, we can
 trust that the tree is immutable and its contents never change.
 
 So you can trust the contents of a tree to be valid, the same way you
 can trust the contents of a blob, but you don't know where those
 contents _came_ from.
 
 Side note on trees: since a "tree" object is a sorted list of
 "filename+content", you can create a diff between two trees without
 actually having to unpack two trees.  Just ignore all common parts,
 and your diff will look right.  In other words, you can effectively
 (and efficiently) tell the difference between any two random trees by
 O(n) where "n" is the size of the difference, rather than the size of
 the tree.
 
 Side note 2 on trees: since the name of a "blob" depends entirely and
 exclusively on its contents (i.e. there are no names or permissions
 involved), you can see trivial renames or permission changes by
 noticing that the blob stayed the same.  However, renames with data
 changes need a smarter "diff" implementation.
 
 A tree is created with link:git-write-tree.html[git-write-tree] and
 it's data can be accessed by link:git-ls-tree.html[git-ls-tree]
 
 Commit Object
 ~~~~~~~~~~~~~
 The "commit" object is an object that introduces the notion of
 history into the picture.  In contrast to the other objects, it
 doesn't just describe the physical state of a tree, it describes how
 we got there, and why.
 
 A "commit" is defined by the tree-object that it results in, the
 parent commits (zero, one or more) that led up to that point, and a
 comment on what happened.  Again, a commit is not trusted per se:
 the contents are well-defined and "safe" due to the cryptographically
 strong signatures at all levels, but there is no reason to believe
 that the tree is "good" or that the merge information makes sense.
 The parents do not have to actually have any relationship with the
 result, for example.
 
 Note on commits: unlike real SCM's, commits do not contain
 rename information or file mode chane information.  All of that is
 implicit in the trees involved (the result tree, and the result trees
 of the parents), and describing that makes no sense in this idiotic
 file manager.
 
 A commit is created with link:git-commit-tree.html[git-commit-tree] and
 it's data can be accessed by link:git-cat-file.html[git-cat-file]
 
 Trust
 ~~~~~
 An aside on the notion of "trust". Trust is really outside the scope
 of "git", but it's worth noting a few things.  First off, since
 everything is hashed with SHA1, you _can_ trust that an object is
 intact and has not been messed with by external sources.  So the name
 of an object uniquely identifies a known state - just not a state that
 you may want to trust.
 
 Furthermore, since the SHA1 signature of a commit refers to the
 SHA1 signatures of the tree it is associated with and the signatures
 of the parent, a single named commit specifies uniquely a whole set
 of history, with full contents.  You can't later fake any step of the
 way once you have the name of a commit.
 
 So to introduce some real trust in the system, the only thing you need
 to do is to digitally sign just _one_ special note, which includes the
 name of a top-level commit.  Your digital signature shows others
 that you trust that commit, and the immutability of the history of
 commits tells others that they can trust the whole history.
 
 In other words, you can easily validate a whole archive by just
 sending out a single email that tells the people the name (SHA1 hash)
 of the top commit, and digitally sign that email using something
 like GPG/PGP.
 
 To assist in this, git also provides the tag object...
 
 Tag Object
 ~~~~~~~~~~
 Git provides the "tag" object to simplify creating, managing and
 exchanging symbolic and signed tokens.  The "tag" object at its
 simplest simply symbolically identifies another object by containing
 the sha1, type and symbolic name.
 
 However it can optionally contain additional signature information
 (which git doesn't care about as long as there's less than 8k of
 it). This can then be verified externally to git.
 
 Note that despite the tag features, "git" itself only handles content
 integrity; the trust framework (and signature provision and
 verification) has to come from outside.
 
 A tag is created with link:git-mktag.html[git-mktag] and
 it's data can be accessed by link:git-cat-file.html[git-cat-file]
 
 +Delta Object
 +~~~~~~~~~~~~
 +
 +The "delta" object is used internally by the object database to
 +minimise storage usage by using xdeltas (byte level diffs). Deltas can
 +form chains of arbitrary length as RCS does (although this is
 +configureable at creation time). Most operations won't see or even be
 +aware of delta objects as they are automatically 'applied' and appear
 +as 'real' git objects In other words, if you write your own routines
 +to look at the contents of the object database then you need to know
 +about this - otherwise you don't. Actually, that's not quite true -
 +one important area where deltas are likely to prove very valuable is
 +in reducing bandwidth loads - so the more sophisticated network tools
 +for git repositories will be aware of them too.
 +
 +Finally, git repositories can (and must) be deltafied in the
 +background - the work to calculate the differences does not take place
 +automatically at commit time.
 +
 +A delta can be created (or undeltafied) with
 +link:git-mkdelta.html[git-mkdelta] it's raw data cannot be accessed at
 +present.
 +
 +
 The "index" aka "Current Directory Cache"
 -----------------------------------------
 The index is a simple binary file, which contains an efficient
 representation of a virtual directory content at some random time.  It
 does so by a simple array that associates a set of names, dates,
 permissions and content (aka "blob") objects together.  The cache is
 always kept ordered by name, and names are unique (with a few very
 specific rules) at any point in time, but the cache has no long-term
 meaning, and can be partially updated at any time.
 
 In particular, the index certainly does not need to be consistent with
 the current directory contents (in fact, most operations will depend on
 different ways to make the index _not_ be consistent with the directory
 hierarchy), but it has three very important attributes:
 
 '(a) it can re-generate the full state it caches (not just the
 directory structure: it contains pointers to the "blob" objects so
 that it can regenerate the data too)'
 
 As a special case, there is a clear and unambiguous one-way mapping
 from a current directory cache to a "tree object", which can be
 efficiently created from just the current directory cache without
 actually looking at any other data.  So a directory cache at any one
 time uniquely specifies one and only one "tree" object (but has
 additional data to make it easy to match up that tree object with what
 has happened in the directory)
 
 '(b) it has efficient methods for finding inconsistencies between that
 cached state ("tree object waiting to be instantiated") and the
 current state.'
 
 '(c) it can additionally efficiently represent information about merge
 conflicts between different tree objects, allowing each pathname to be
 associated with sufficient information about the trees involved that
 you can create a three-way merge between them.'
 
 Those are the three ONLY things that the directory cache does.  It's a
 cache, and the normal operation is to re-generate it completely from a
 known tree object, or update/compare it with a live tree that is being
 developed.  If you blow the directory cache away entirely, you generally
 haven't lost any information as long as you have the name of the tree
 that it described. 
 
 At the same time, the directory index is at the same time also the
 staging area for creating new trees, and creating a new tree always
 involves a controlled modification of the index file.  In particular,
 the index file can have the representation of an intermediate tree that
 has not yet been instantiated.  So the index can be thought of as a
 write-back cache, which can contain dirty information that has not yet
 been written back to the backing store.
 
 
 
 The Workflow
 ------------
 Generally, all "git" operations work on the index file. Some operations
 work *purely* on the index file (showing the current state of the
 index), but most operations move data to and from the index file. Either
 from the database or from the working directory. Thus there are four
 main combinations: 
 
 1) working directory -> index
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You update the index with information from the working directory with
 the link:git-update-cache.html[git-update-cache] command.  You
 generally update the index information by just specifying the filename
 you want to update, like so:
 
 	git-update-cache filename
 
 but to avoid common mistakes with filename globbing etc, the command
 will not normally add totally new entries or remove old entries,
 i.e. it will normally just update existing cache entries.
 
 To tell git that yes, you really do realize that certain files no
 longer exist in the archive, or that new files should be added, you
 should use the "--remove" and "--add" flags respectively.
 
 NOTE! A "--remove" flag does _not_ mean that subsequent filenames will
 necessarily be removed: if the files still exist in your directory
 structure, the index will be updated with their new status, not
 removed. The only thing "--remove" means is that update-cache will be
 considering a removed file to be a valid thing, and if the file really
 does not exist any more, it will update the index accordingly.
 
 As a special case, you can also do "git-update-cache --refresh", which
 will refresh the "stat" information of each index to match the current
 stat information. It will _not_ update the object status itself, and
 it will only update the fields that are used to quickly test whether
 an object still matches its old backing store object.
 
 2) index -> object database
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You write your current index file to a "tree" object with the program
 
 	git-write-tree
 
 that doesn't come with any options - it will just write out the
 current index into the set of tree objects that describe that state,
 and it will return the name of the resulting top-level tree. You can
 use that tree to re-generate the index at any time by going in the
 other direction:
 
 3) object database -> index
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You read a "tree" file from the object database, and use that to
 populate (and overwrite - don't do this if your index contains any
 unsaved state that you might want to restore later!) your current
 index.  Normal operation is just
 
 		git-read-tree <sha1 of tree>
 
 and your index file will now be equivalent to the tree that you saved
 earlier. However, that is only your _index_ file: your working
 directory contents have not been modified.
 
 4) index -> working directory
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You update your working directory from the index by "checking out"
 files. This is not a very common operation, since normally you'd just
 keep your files updated, and rather than write to your working
 directory, you'd tell the index files about the changes in your
 working directory (i.e. "git-update-cache").
 
 However, if you decide to jump to a new version, or check out somebody
 else's version, or just restore a previous tree, you'd populate your
 index file with read-tree, and then you need to check out the result
 with
 		git-checkout-cache filename
 
 or, if you want to check out all of the index, use "-a".
 
 NOTE! git-checkout-cache normally refuses to overwrite old files, so
 if you have an old version of the tree already checked out, you will
 need to use the "-f" flag (_before_ the "-a" flag or the filename) to
 _force_ the checkout.
 
 
 Finally, there are a few odds and ends which are not purely moving
 from one representation to the other:
 
 5) Tying it all together
 ~~~~~~~~~~~~~~~~~~~~~~~~
 To commit a tree you have instantiated with "git-write-tree", you'd
 create a "commit" object that refers to that tree and the history
 behind it - most notably the "parent" commits that preceded it in
 history.
 
 Normally a "commit" has one parent: the previous state of the tree
 before a certain change was made. However, sometimes it can have two
 or more parent commits, in which case we call it a "merge", due to the
 fact that such a commit brings together ("merges") two or more
 previous states represented by other commits.
 
 In other words, while a "tree" represents a particular directory state
 of a working directory, a "commit" represents that state in "time",
 and explains how we got there.
 
 You create a commit object by giving it the tree that describes the
 state at the time of the commit, and a list of parents:
 
 	git-commit-tree <tree> -p <parent> [-p <parent2> ..]
 
 and then giving the reason for the commit on stdin (either through
 redirection from a pipe or file, or by just typing it at the tty).
 
 git-commit-tree will return the name of the object that represents
 that commit, and you should save it away for later use. Normally,
 you'd commit a new "HEAD" state, and while git doesn't care where you
 save the note about that state, in practice we tend to just write the
 result to the file ".git/HEAD", so that we can always see what the
 last committed state was.
 
 6) Examining the data
 ~~~~~~~~~~~~~~~~~~~~~
 
 You can examine the data represented in the object database and the
 index with various helper tools. For every object, you can use
 link:git-cat-file.html[git-cat-file] to examine details about the
 object:
 
 		git-cat-file -t <objectname>
 
 shows the type of the object, and once you have the type (which is
 usually implicit in where you find the object), you can use
 
 		git-cat-file blob|tree|commit <objectname>
 
 to show its contents. NOTE! Trees have binary content, and as a result
 there is a special helper for showing that content, called
 "git-ls-tree", which turns the binary content into a more easily
 readable form.
 
 It's especially instructive to look at "commit" objects, since those
 tend to be small and fairly self-explanatory. In particular, if you
 follow the convention of having the top commit name in ".git/HEAD",
 you can do
 
 		git-cat-file commit $(cat .git/HEAD)
 
 to see what the top commit was.
 
 7) Merging multiple trees
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Git helps you do a three-way merge, which you can expand to n-way by
 repeating the merge procedure arbitrary times until you finally
 "commit" the state.  The normal situation is that you'd only do one
 three-way merge (two parents), and commit it, but if you like to, you
 can do multiple parents in one go.
 
 To do a three-way merge, you need the two sets of "commit" objects
 that you want to merge, use those to find the closest common parent (a
 third "commit" object), and then use those commit objects to find the
 state of the directory ("tree" object) at these points.
 
 To get the "base" for the merge, you first look up the common parent
 of two commits with
 
 		git-merge-base <commit1> <commit2>
 
 which will return you the commit they are both based on.  You should
 now look up the "tree" objects of those commits, which you can easily
 do with (for example)
 
 		git-cat-file commit <commitname> | head -1
 
 since the tree object information is always the first line in a commit
 object.
 
 Once you know the three trees you are going to merge (the one
 "original" tree, aka the common case, and the two "result" trees, aka
 the branches you want to merge), you do a "merge" read into the
 index. This will throw away your old index contents, so you should
 make sure that you've committed those - in fact you would normally
 always do a merge against your last commit (which should thus match
 what you have in your current index anyway).
 
 To do the merge, do
 
 		git-read-tree -m <origtree> <target1tree> <target2tree>
 
 which will do all trivial merge operations for you directly in the
 index file, and you can just write the result out with
 "git-write-tree".
 
 NOTE! Because the merge is done in the index file, and not in your
 working directory, your working directory will no longer match your
 index. You can use "git-checkout-cache -f -a" to make the effect of
 the merge be seen in your working directory.
 
 NOTE2! Sadly, many merges aren't trivial. If there are files that have
 been added.moved or removed, or if both branches have modified the
 same file, you will be left with an index tree that contains "merge
 entries" in it. Such an index tree can _NOT_ be written out to a tree
 object, and you will have to resolve any such merge clashes using
 other tools before you can write out the result.
 
 
 [ fixme: talk about resolving merges here ]

 commit 7096a645cde91c96ce849c43750c04433164418c
 Author: David Greaves <[email protected]>
 Date:   Sun May 22 18:44:17 2005 +0100

    [PATCH] Docs - tag object, git- prefix and s/changeset/commit/g
    
    Add docs for tag type
    Rename commands to have git- prefix
    Rename changeset to commit throughout
    
    Signed-off-by: David Greaves <[email protected]>
    Signed-off-by: Linus Torvalds <[email protected]>

 diff --git a/README b/README
 --- a/README
 +++ b/README
 @@ -17,446 +17,465 @@
 This is a stupid (but extremely fast) directory content manager.  It
 doesn't do a whole lot, but what it _does_ do is track directory
 contents efficiently. 
 
 There are two object abstractions: the "object database", and the
 "current directory cache" aka "index".
 
 The Object Database
 ~~~~~~~~~~~~~~~~~~~
 The object database is literally just a content-addressable collection
 of objects.  All objects are named by their content, which is
 approximated by the SHA1 hash of the object itself.  Objects may refer
 to other objects (by referencing their SHA1 hash), and so you can
 build up a hierarchy of objects.
 
 All objects have a statically determined "type" aka "tag", which is
 determined at object creation time, and which identifies the format of
 -the object (i.e. how it is used, and how it can refer to other objects). 
 -There are currently three different object types: "blob", "tree" and
 -"commit". 
 +the object (i.e. how it is used, and how it can refer to other
 +objects).  There are currently four different object types: "blob",
 +"tree", "commit" and "tag".
 
 A "blob" object cannot refer to any other object, and is, like the tag
 implies, a pure storage object containing some user data.  It is used to
 actually store the file data, i.e. a blob object is associated with some
 particular version of some file. 
 
 A "tree" object is an object that ties one or more "blob" objects into a
 directory structure. In addition, a tree object can refer to other tree
 objects, thus creating a directory hierarchy. 
 
 -Finally, a "commit" object ties such directory hierarchies together into
 +A "commit" object ties such directory hierarchies together into
 a DAG of revisions - each "commit" is associated with exactly one tree
 (the directory hierarchy at the time of the commit). In addition, a
 "commit" refers to one or more "parent" commit objects that describe the
 history of how we arrived at that directory hierarchy.
 
 As a special case, a commit object with no parents is called the "root"
 object, and is the point of an initial project commit.  Each project
 must have at least one root, and while you can tie several different
 root objects together into one project by creating a commit object which
 has two or more separate roots as its ultimate parents, that's probably
 just going to confuse people.  So aim for the notion of "one root object
 per project", even if git itself does not enforce that. 
 
 A "tag" object symbolically identifies and can be used to sign other
 objects. It contains the identifier and type of another object, a
 symbolic name (of course!) and, optionally, a signature.
 
 Regardless of object type, all objects are share the following
 characteristics: they are all in deflated with zlib, and have a header
 that not only specifies their tag, but also size information about the
 data in the object.  It's worth noting that the SHA1 hash that is used
 to name the object is the hash of the original data (historical note:
 in the dawn of the age of git this was the sha1 of the _compressed_
 object)
 
 As a result, the general consistency of an object can always be tested
 independently of the contents or the type of the object: all objects can
 be validated by verifying that (a) their hashes match the content of the
 file and (b) the object successfully inflates to a stream of bytes that
 forms a sequence of <ascii tag without space> + <space> + <ascii decimal
 size> + <byte\0> + <binary object data>. 
 
 The structured objects can further have their structure and
 connectivity to other objects verified. This is generally done with
 -the "fsck-cache" program, which generates a full dependency graph of
 -all objects, and verifies their internal consistency (in addition to
 -just verifying their superficial consistency through the hash).
 +the "git-fsck-cache" program, which generates a full dependency graph
 +of all objects, and verifies their internal consistency (in addition
 +to just verifying their superficial consistency through the hash).
 
 The object types in some more detail:
 
 Blob Object
 ~~~~~~~~~~~
 A "blob" object is nothing but a binary blob of data, and doesn't
 refer to anything else.  There is no signature or any other
 verification of the data, so while the object is consistent (it _is_
 indexed by its sha1 hash, so the data itself is certainly correct), it
 has absolutely no other attributes.  No name associations, no
 permissions.  It is purely a blob of data (i.e. normally "file
 contents").
 
 In particular, since the blob is entirely defined by its data, if two
 files in a directory tree (or in multiple different versions of the
 repository) have the same contents, they will share the same blob
 object. The object is totally independent of it's location in the
 directory tree, and renaming a file does not change the object that
 file is associated with in any way.
 
 +A blob is created with link:git-write-blob.html[git-write-blob] and
 +it's data can be accessed by link:git-cat-file.html[git-cat-file]
 +
 Tree Object
 ~~~~~~~~~~~
 The next hierarchical object type is the "tree" object.  A tree object
 is a list of mode/name/blob data, sorted by name.  Alternatively, the
 mode data may specify a directory mode, in which case instead of
 naming a blob, that name is associated with another TREE object.
 
 Like the "blob" object, a tree object is uniquely determined by the
 set contents, and so two separate but identical trees will always
 share the exact same object. This is true at all levels, i.e. it's
 true for a "leaf" tree (which does not refer to any other trees, only
 blobs) as well as for a whole subdirectory.
 
 For that reason a "tree" object is just a pure data abstraction: it
 has no history, no signatures, no verification of validity, except
 that since the contents are again protected by the hash itself, we can
 trust that the tree is immutable and its contents never change.
 
 So you can trust the contents of a tree to be valid, the same way you
 can trust the contents of a blob, but you don't know where those
 contents _came_ from.
 
 Side note on trees: since a "tree" object is a sorted list of
 "filename+content", you can create a diff between two trees without
 actually having to unpack two trees.  Just ignore all common parts,
 and your diff will look right.  In other words, you can effectively
 (and efficiently) tell the difference between any two random trees by
 O(n) where "n" is the size of the difference, rather than the size of
 the tree.
 
 Side note 2 on trees: since the name of a "blob" depends entirely and
 exclusively on its contents (i.e. there are no names or permissions
 involved), you can see trivial renames or permission changes by
 noticing that the blob stayed the same.  However, renames with data
 changes need a smarter "diff" implementation.
 
 +A tree is created with link:git-write-tree.html[git-write-tree] and
 +it's data can be accessed by link:git-ls-tree.html[git-ls-tree]
 
 -Changeset Object
 -~~~~~~~~~~~~~~~~
 -The "changeset" object is an object that introduces the notion of
 +Commit Object
 +~~~~~~~~~~~~~
 +The "commit" object is an object that introduces the notion of
 history into the picture.  In contrast to the other objects, it
 doesn't just describe the physical state of a tree, it describes how
 we got there, and why.
 
 -A "changeset" is defined by the tree-object that it results in, the
 -parent changesets (zero, one or more) that led up to that point, and a
 -comment on what happened.  Again, a changeset is not trusted per se:
 +A "commit" is defined by the tree-object that it results in, the
 +parent commits (zero, one or more) that led up to that point, and a
 +comment on what happened.  Again, a commit is not trusted per se:
 the contents are well-defined and "safe" due to the cryptographically
 strong signatures at all levels, but there is no reason to believe
 that the tree is "good" or that the merge information makes sense.
 The parents do not have to actually have any relationship with the
 result, for example.
 
 -Note on changesets: unlike real SCM's, changesets do not contain
 -rename information or file mode change information.  All of that is
 +Note on commits: unlike real SCM's, commits do not contain
 +rename information or file mode chane information.  All of that is
 implicit in the trees involved (the result tree, and the result trees
 of the parents), and describing that makes no sense in this idiotic
 file manager.
 
 -Trust Object
 -~~~~~~~~~~~~
 -The notion of "trust" is really outside the scope of "git", but it's
 -worth noting a few things.  First off, since everything is hashed with
 -SHA1, you _can_ trust that an object is intact and has not been messed
 -with by external sources.  So the name of an object uniquely
 -identifies a known state - just not a state that you may want to
 -trust.
 +A commit is created with link:git-commit-tree.html[git-commit-tree] and
 +it's data can be accessed by link:git-cat-file.html[git-cat-file]
 +
 +Trust
 +~~~~~
 +An aside on the notion of "trust". Trust is really outside the scope
 +of "git", but it's worth noting a few things.  First off, since
 +everything is hashed with SHA1, you _can_ trust that an object is
 +intact and has not been messed with by external sources.  So the name
 +of an object uniquely identifies a known state - just not a state that
 +you may want to trust.
 
 -Furthermore, since the SHA1 signature of a changeset refers to the
 +Furthermore, since the SHA1 signature of a commit refers to the
 SHA1 signatures of the tree it is associated with and the signatures
 -of the parent, a single named changeset specifies uniquely a whole set
 +of the parent, a single named commit specifies uniquely a whole set
 of history, with full contents.  You can't later fake any step of the
 -way once you have the name of a changeset.
 +way once you have the name of a commit.
 
 So to introduce some real trust in the system, the only thing you need
 to do is to digitally sign just _one_ special note, which includes the
 -name of a top-level changeset.  Your digital signature shows others
 -that you trust that changeset, and the immutability of the history of
 -changesets tells others that they can trust the whole history.
 +name of a top-level commit.  Your digital signature shows others
 +that you trust that commit, and the immutability of the history of
 +commits tells others that they can trust the whole history.
 
 In other words, you can easily validate a whole archive by just
 sending out a single email that tells the people the name (SHA1 hash)
 -of the top changeset, and digitally sign that email using something
 +of the top commit, and digitally sign that email using something
 like GPG/PGP.
 
 -In particular, you can also have a separate archive of "trust points"
 -or tags, which document your (and other peoples) trust.  You may, of
 -course, archive these "certificates of trust" using "git" itself, but
 -it's not something "git" does for you.
 +To assist in this, git also provides the tag object...
 
 -Another way of saying the last point: "git" itself only handles
 -content integrity, the trust has to come from outside.
 +Tag Object
 +~~~~~~~~~~
 +Git provides the "tag" object to simplify creating, managing and
 +exchanging symbolic and signed tokens.  The "tag" object at its
 +simplest simply symbolically identifies another object by containing
 +the sha1, type and symbolic name.
 
 +However it can optionally contain additional signature information
 +(which git doesn't care about as long as there's less than 8k of
 +it). This can then be verified externally to git.
 
 +Note that despite the tag features, "git" itself only handles content
 +integrity; the trust framework (and signature provision and
 +verification) has to come from outside.
 
 +A tag is created with link:git-mktag.html[git-mktag] and
 +it's data can be accessed by link:git-cat-file.html[git-cat-file]
 
 The "index" aka "Current Directory Cache"
 -----------------------------------------
 The index is a simple binary file, which contains an efficient
 representation of a virtual directory content at some random time.  It
 does so by a simple array that associates a set of names, dates,
 permissions and content (aka "blob") objects together.  The cache is
 always kept ordered by name, and names are unique (with a few very
 specific rules) at any point in time, but the cache has no long-term
 meaning, and can be partially updated at any time.
 
 In particular, the index certainly does not need to be consistent with
 the current directory contents (in fact, most operations will depend on
 different ways to make the index _not_ be consistent with the directory
 hierarchy), but it has three very important attributes:
 
 '(a) it can re-generate the full state it caches (not just the
 directory structure: it contains pointers to the "blob" objects so
 that it can regenerate the data too)'
 
 As a special case, there is a clear and unambiguous one-way mapping
 from a current directory cache to a "tree object", which can be
 efficiently created from just the current directory cache without
 actually looking at any other data.  So a directory cache at any one
 time uniquely specifies one and only one "tree" object (but has
 additional data to make it easy to match up that tree object with what
 has happened in the directory)
 
 '(b) it has efficient methods for finding inconsistencies between that
 cached state ("tree object waiting to be instantiated") and the
 current state.'
 
 '(c) it can additionally efficiently represent information about merge
 conflicts between different tree objects, allowing each pathname to be
 associated with sufficient information about the trees involved that
 you can create a three-way merge between them.'
 
 Those are the three ONLY things that the directory cache does.  It's a
 cache, and the normal operation is to re-generate it completely from a
 known tree object, or update/compare it with a live tree that is being
 developed.  If you blow the directory cache away entirely, you generally
 haven't lost any information as long as you have the name of the tree
 that it described. 
 
 At the same time, the directory index is at the same time also the
 staging area for creating new trees, and creating a new tree always
 involves a controlled modification of the index file.  In particular,
 the index file can have the representation of an intermediate tree that
 has not yet been instantiated.  So the index can be thought of as a
 write-back cache, which can contain dirty information that has not yet
 been written back to the backing store.
 
 
 
 The Workflow
 ------------
 Generally, all "git" operations work on the index file. Some operations
 work *purely* on the index file (showing the current state of the
 index), but most operations move data to and from the index file. Either
 from the database or from the working directory. Thus there are four
 main combinations: 
 
 1) working directory -> index
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You update the index with information from the working directory with
 -the "update-cache" command.  You generally update the index
 -information by just specifying the filename you want to update, like
 -so:
 +the link:git-update-cache.html[git-update-cache] command.  You
 +generally update the index information by just specifying the filename
 +you want to update, like so:
 
 -		update-cache filename
 +	git-update-cache filename
 
 but to avoid common mistakes with filename globbing etc, the command
 will not normally add totally new entries or remove old entries,
 i.e. it will normally just update existing cache entries.
 
 To tell git that yes, you really do realize that certain files no
 longer exist in the archive, or that new files should be added, you
 should use the "--remove" and "--add" flags respectively.
 
 NOTE! A "--remove" flag does _not_ mean that subsequent filenames will
 necessarily be removed: if the files still exist in your directory
 structure, the index will be updated with their new status, not
 removed. The only thing "--remove" means is that update-cache will be
 considering a removed file to be a valid thing, and if the file really
 does not exist any more, it will update the index accordingly.
 
 -As a special case, you can also do "update-cache --refresh", which
 +As a special case, you can also do "git-update-cache --refresh", which
 will refresh the "stat" information of each index to match the current
 stat information. It will _not_ update the object status itself, and
 it will only update the fields that are used to quickly test whether
 an object still matches its old backing store object.
 
 2) index -> object database
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You write your current index file to a "tree" object with the program
 
 -		write-tree
 +	git-write-tree
 
 that doesn't come with any options - it will just write out the
 current index into the set of tree objects that describe that state,
 and it will return the name of the resulting top-level tree. You can
 use that tree to re-generate the index at any time by going in the
 other direction:
 
 3) object database -> index
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You read a "tree" file from the object database, and use that to
 populate (and overwrite - don't do this if your index contains any
 unsaved state that you might want to restore later!) your current
 index.  Normal operation is just
 
 -		read-tree <sha1 of tree>
 +		git-read-tree <sha1 of tree>
 
 and your index file will now be equivalent to the tree that you saved
 earlier. However, that is only your _index_ file: your working
 directory contents have not been modified.
 
 4) index -> working directory
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 You update your working directory from the index by "checking out"
 files. This is not a very common operation, since normally you'd just
 keep your files updated, and rather than write to your working
 directory, you'd tell the index files about the changes in your
 -working directory (i.e. "update-cache").
 +working directory (i.e. "git-update-cache").
 
 However, if you decide to jump to a new version, or check out somebody
 else's version, or just restore a previous tree, you'd populate your
 index file with read-tree, and then you need to check out the result
 with
 -
 -		checkout-cache filename
 +		git-checkout-cache filename
 
 or, if you want to check out all of the index, use "-a".
 
 -NOTE! checkout-cache normally refuses to overwrite old files, so if
 -you have an old version of the tree already checked out, you will need
 -to use the "-f" flag (_before_ the "-a" flag or the filename) to
 +NOTE! git-checkout-cache normally refuses to overwrite old files, so
 +if you have an old version of the tree already checked out, you will
 +need to use the "-f" flag (_before_ the "-a" flag or the filename) to
 _force_ the checkout.
 
 
 Finally, there are a few odds and ends which are not purely moving
 from one representation to the other:
 
 5) Tying it all together
 ~~~~~~~~~~~~~~~~~~~~~~~~
 -
 -To commit a tree you have instantiated with "write-tree", you'd create
 -a "commit" object that refers to that tree and the history behind it -
 -most notably the "parent" commits that preceded it in history.
 +To commit a tree you have instantiated with "git-write-tree", you'd
 +create a "commit" object that refers to that tree and the history
 +behind it - most notably the "parent" commits that preceded it in
 +history.
 
 Normally a "commit" has one parent: the previous state of the tree
 before a certain change was made. However, sometimes it can have two
 or more parent commits, in which case we call it a "merge", due to the
 fact that such a commit brings together ("merges") two or more
 previous states represented by other commits.
 
 In other words, while a "tree" represents a particular directory state
 of a working directory, a "commit" represents that state in "time",
 and explains how we got there.
 
 You create a commit object by giving it the tree that describes the
 state at the time of the commit, and a list of parents:
 
 -		commit-tree <tree> -p <parent> [-p <parent2> ..]
 +	git-commit-tree <tree> -p <parent> [-p <parent2> ..]
 
 and then giving the reason for the commit on stdin (either through
 redirection from a pipe or file, or by just typing it at the tty).
 
 -commit-tree will return the name of the object that represents that
 -commit, and you should save it away for later use. Normally, you'd
 -commit a new "HEAD" state, and while git doesn't care where you save
 -the note about that state, in practice we tend to just write the
 +git-commit-tree will return the name of the object that represents
 +that commit, and you should save it away for later use. Normally,
 +you'd commit a new "HEAD" state, and while git doesn't care where you
 +save the note about that state, in practice we tend to just write the
 result to the file ".git/HEAD", so that we can always see what the
 last committed state was.
 
 6) Examining the data
 ~~~~~~~~~~~~~~~~~~~~~
 
 You can examine the data represented in the object database and the
 index with various helper tools. For every object, you can use
 -"cat-file" to examine details about the object:
 +link:git-cat-file.html[git-cat-file] to examine details about the
 +object:
 
 -		cat-file -t <objectname>
 +		git-cat-file -t <objectname>
 
 shows the type of the object, and once you have the type (which is
 usually implicit in where you find the object), you can use
 
 -		cat-file blob|tree|commit <objectname>
 +		git-cat-file blob|tree|commit <objectname>
 
 to show its contents. NOTE! Trees have binary content, and as a result
 -there is a special helper for showing that content, called "ls-tree",
 -which turns the binary content into a more easily readable form.
 +there is a special helper for showing that content, called
 +"git-ls-tree", which turns the binary content into a more easily
 +readable form.
 
 It's especially instructive to look at "commit" objects, since those
 tend to be small and fairly self-explanatory. In particular, if you
 follow the convention of having the top commit name in ".git/HEAD",
 you can do
 
 -		cat-file commit $(cat .git/HEAD)
 +		git-cat-file commit $(cat .git/HEAD)
 
 to see what the top commit was.
 
 7) Merging multiple trees
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Git helps you do a three-way merge, which you can expand to n-way by
 repeating the merge procedure arbitrary times until you finally
 "commit" the state.  The normal situation is that you'd only do one
 three-way merge (two parents), and commit it, but if you like to, you
 can do multiple parents in one go.
 
 To do a three-way merge, you need the two sets of "commit" objects
 that you want to merge, use those to find the closest common parent (a
 third "commit" object), and then use those commit objects to find the
 state of the directory ("tree" object) at these points.
 
 To get the "base" for the merge, you first look up the common parent
 of two commits with
 
 -		merge-base <commit1> <commit2>
 +		git-merge-base <commit1> <commit2>
 
 which will return you the commit they are both based on.  You should
 now look up the "tree" objects of those commits, which you can easily
 do with (for example)
 
 -		cat-file commit <commitname> | head -1
 +		git-cat-file commit <commitname> | head -1
 
 since the tree object information is always the first line in a commit
 object.
 
 Once you know the three trees you are going to merge (the one
 "original" tree, aka the common case, and the two "result" trees, aka
 the branches you want to merge), you do a "merge" read into the
 index. This will throw away your old index contents, so you should
 make sure that you've committed those - in fact you would normally
 always do a merge against your last commit (which should thus match
 what you have in your current index anyway).
 
 To do the merge, do
 
 -		read-tree -m <origtree> <target1tree> <target2tree>
 +		git-read-tree -m <origtree> <target1tree> <target2tree>
 
 which will do all trivial merge operations for you directly in the
 -index file, and you can just write the result out with "write-tree".
 +index file, and you can just write the result out with
 +"git-write-tree".
 
 NOTE! Because the merge is done in the index file, and not in your
 working directory, your working directory will no longer match your
 -index. You can use "checkout-cache -f -a" to make the effect of the
 -merge be seen in your working directory.
 +index. You can use "git-checkout-cache -f -a" to make the effect of
 +the merge be seen in your working directory.
 
 NOTE2! Sadly, many merges aren't trivial. If there are files that have
 been added.moved or removed, or if both branches have modified the
 same file, you will be left with an index tree that contains "merge
 entries" in it. Such an index tree can _NOT_ be written out to a tree
 object, and you will have to resolve any such merge clashes using
 other tools before you can write out the result.
 
 
 [ fixme: talk about resolving merges here ]

 commit 8ac866a869a61d382486ace6ea39f9741d9159f8
 Author: David Greaves <[email protected]>
 Date:   Sun May 22 18:44:16 2005 +0100

    [PATCH] Docs - asciidoc changes
    
    Whitespace and asciidoc formatting changes only in preparation for
    content changes.
    
    Signed-off-by: David Greaves <[email protected]>
    Signed-off-by: Linus Torvalds <[email protected]>

 diff --git a/README b/README
 --- a/README
 +++ b/README
 @@ -18,443 +17,446 @@
 This is a stupid (but extremely fast) directory content manager.  It
 doesn't do a whole lot, but what it _does_ do is track directory
 contents efficiently. 
 
 There are two object abstractions: the "object database", and the
 "current directory cache" aka "index".
 
 -
 -
 -	The Object Database (GIT_OBJECT_DIRECTORY)
 -
 -
 +The Object Database
 +~~~~~~~~~~~~~~~~~~~
 The object database is literally just a content-addressable collection
 of objects.  All objects are named by their content, which is
 approximated by the SHA1 hash of the object itself.  Objects may refer
 -to other objects (by referencing their SHA1 hash), and so you can build
 -up a hierarchy of objects. 
 +to other objects (by referencing their SHA1 hash), and so you can
 +build up a hierarchy of objects.
 
 All objects have a statically determined "type" aka "tag", which is
 determined at object creation time, and which identifies the format of
 the object (i.e. how it is used, and how it can refer to other objects). 
 There are currently three different object types: "blob", "tree" and
 "commit". 
 
 A "blob" object cannot refer to any other object, and is, like the tag
 implies, a pure storage object containing some user data.  It is used to
 actually store the file data, i.e. a blob object is associated with some
 particular version of some file. 
 
 A "tree" object is an object that ties one or more "blob" objects into a
 directory structure. In addition, a tree object can refer to other tree
 objects, thus creating a directory hierarchy. 
 
 Finally, a "commit" object ties such directory hierarchies together into
 a DAG of revisions - each "commit" is associated with exactly one tree
 (the directory hierarchy at the time of the commit). In addition, a
 "commit" refers to one or more "parent" commit objects that describe the
 history of how we arrived at that directory hierarchy.
 
 As a special case, a commit object with no parents is called the "root"
 object, and is the point of an initial project commit.  Each project
 must have at least one root, and while you can tie several different
 root objects together into one project by creating a commit object which
 has two or more separate roots as its ultimate parents, that's probably
 just going to confuse people.  So aim for the notion of "one root object
 per project", even if git itself does not enforce that. 
 
 +A "tag" object symbolically identifies and can be used to sign other
 +objects. It contains the identifier and type of another object, a
 +symbolic name (of course!) and, optionally, a signature.
 +
 Regardless of object type, all objects are share the following
 characteristics: they are all in deflated with zlib, and have a header
 that not only specifies their tag, but also size information about the
 data in the object.  It's worth noting that the SHA1 hash that is used
 -to name the object is always the hash of this _compressed_ object, not
 -the original data.
 +to name the object is the hash of the original data (historical note:
 +in the dawn of the age of git this was the sha1 of the _compressed_
 +object)
 
 As a result, the general consistency of an object can always be tested
 independently of the contents or the type of the object: all objects can
 be validated by verifying that (a) their hashes match the content of the
 file and (b) the object successfully inflates to a stream of bytes that
 forms a sequence of <ascii tag without space> + <space> + <ascii decimal
 size> + <byte\0> + <binary object data>. 
 
 -The structured objects can further have their structure and connectivity
 -to other objects verified. This is generally done with the "fsck-cache"
 -program, which generates a full dependency graph of all objects, and
 -verifies their internal consistency (in addition to just verifying their
 -superficial consistency through the hash).
 +The structured objects can further have their structure and
 +connectivity to other objects verified. This is generally done with
 +the "fsck-cache" program, which generates a full dependency graph of
 +all objects, and verifies their internal consistency (in addition to
 +just verifying their superficial consistency through the hash).
 
 The object types in some more detail:
 
 -  BLOB: A "blob" object is nothing but a binary blob of data, and
 -	doesn't refer to anything else.  There is no signature or any
 -	other verification of the data, so while the object is
 -	consistent (it _is_ indexed by its sha1 hash, so the data itself
 -	is certainly correct), it has absolutely no other attributes. 
 -	No name associations, no permissions.  It is purely a blob of
 -	data (i.e. normally "file contents"). 
 -
 -	In particular, since the blob is entirely defined by its data,
 -	if two files in a directory tree (or in multiple different
 -	versions of the repository) have the same contents, they will
 -	share the same blob object. The object is totally independent
 -	of it's location in the directory tree, and renaming a file does
 -	not change the object that file is associated with in any way.
 -
 -  TREE: The next hierarchical object type is the "tree" object.  A tree
 -	object is a list of mode/name/blob data, sorted by name. 
 -	Alternatively, the mode data may specify a directory mode, in
 -	which case instead of naming a blob, that name is associated
 -	with another TREE object. 
 -
 -	Like the "blob" object, a tree object is uniquely determined by
 -	the set contents, and so two separate but identical trees will
 -	always share the exact same object. This is true at all levels,
 -	i.e. it's true for a "leaf" tree (which does not refer to any
 -	other trees, only blobs) as well as for a whole subdirectory.
 -
 -	For that reason a "tree" object is just a pure data abstraction:
 -	it has no history, no signatures, no verification of validity,
 -	except that since the contents are again protected by the hash
 -	itself, we can trust that the tree is immutable and its contents
 -	never change. 
 -
 -	So you can trust the contents of a tree to be valid, the same
 -	way you can trust the contents of a blob, but you don't know
 -	where those contents _came_ from.
 -
 -	Side note on trees: since a "tree" object is a sorted list of
 -	"filename+content", you can create a diff between two trees
 -	without actually having to unpack two trees.  Just ignore all
 -	common parts, and your diff will look right.  In other words,
 -	you can effectively (and efficiently) tell the difference
 -	between any two random trees by O(n) where "n" is the size of
 -	the difference, rather than the size of the tree. 
 -
 -	Side note 2 on trees: since the name of a "blob" depends
 -	entirely and exclusively on its contents (i.e. there are no names
 -	or permissions involved), you can see trivial renames or
 -	permission changes by noticing that the blob stayed the same. 
 -	However, renames with data changes need a smarter "diff" implementation. 
 -
 -CHANGESET: The "changeset" object is an object that introduces the
 -	notion of history into the picture.  In contrast to the other
 -	objects, it doesn't just describe the physical state of a tree,
 -	it describes how we got there, and why. 
 -
 -	A "changeset" is defined by the tree-object that it results in,
 -	the parent changesets (zero, one or more) that led up to that
 -	point, and a comment on what happened.  Again, a changeset is
 -	not trusted per se: the contents are well-defined and "safe" due
 -	to the cryptographically strong signatures at all levels, but
 -	there is no reason to believe that the tree is "good" or that
 -	the merge information makes sense.  The parents do not have to
 -	actually have any relationship with the result, for example. 
 -
 -	Note on changesets: unlike real SCM's, changesets do not contain
 -	rename information or file mode change information.  All of that
 -	is implicit in the trees involved (the result tree, and the
 -	result trees of the parents), and describing that makes no sense
 -	in this idiotic file manager. 
 -
 -TRUST: The notion of "trust" is really outside the scope of "git", but
 -	it's worth noting a few things.  First off, since everything is
 -	hashed with SHA1, you _can_ trust that an object is intact and
 -	has not been messed with by external sources.  So the name of an
 -	object uniquely identifies a known state - just not a state that
 -	you may want to trust. 
 -
 -	Furthermore, since the SHA1 signature of a changeset refers to
 -	the SHA1 signatures of the tree it is associated with and the
 -	signatures of the parent, a single named changeset specifies
 -	uniquely a whole set of history, with full contents.  You can't
 -	later fake any step of the way once you have the name of a
 -	changeset. 
 -
 -	So to introduce some real trust in the system, the only thing
 -	you need to do is to digitally sign just _one_ special note,
 -	which includes the name of a top-level changeset.  Your digital
 -	signature shows others that you trust that changeset, and the
 -	immutability of the history of changesets tells others that they
 -	can trust the whole history. 
 -
 -	In other words, you can easily validate a whole archive by just
 -	sending out a single email that tells the people the name (SHA1
 -	hash) of the top changeset, and digitally sign that email using
 -	something like GPG/PGP. 
 -
 -	In particular, you can also have a separate archive of "trust
 -	points" or tags, which document your (and other peoples) trust. 
 -	You may, of course, archive these "certificates of trust" using
 -	"git" itself, but it's not something "git" does for you. 
 -
 -Another way of saying the last point: "git" itself only handles content
 -integrity, the trust has to come from outside. 
 -
 -
 -
 -	The "index" aka "Current Directory Cache" (".git/index")
 -
 -
 +Blob Object
 +~~~~~~~~~~~
 +A "blob" object is nothing but a binary blob of data, and doesn't
 +refer to anything else.  There is no signature or any other
 +verification of the data, so while the object is consistent (it _is_
 +indexed by its sha1 hash, so the data itself is certainly correct), it
 +has absolutely no other attributes.  No name associations, no
 +permissions.  It is purely a blob of data (i.e. normally "file
 +contents").
 +
 +In particular, since the blob is entirely defined by its data, if two
 +files in a directory tree (or in multiple different versions of the
 +repository) have the same contents, they will share the same blob
 +object. The object is totally independent of it's location in the
 +directory tree, and renaming a file does not change the object that
 +file is associated with in any way.
 +
 +Tree Object
 +~~~~~~~~~~~
 +The next hierarchical object type is the "tree" object.  A tree object
 +is a list of mode/name/blob data, sorted by name.  Alternatively, the
 +mode data may specify a directory mode, in which case instead of
 +naming a blob, that name is associated with another TREE object.
 +
 +Like the "blob" object, a tree object is uniquely determined by the
 +set contents, and so two separate but identical trees will always
 +share the exact same object. This is true at all levels, i.e. it's
 +true for a "leaf" tree (which does not refer to any other trees, only
 +blobs) as well as for a whole subdirectory.
 +
 +For that reason a "tree" object is just a pure data abstraction: it
 +has no history, no signatures, no verification of validity, except
 +that since the contents are again protected by the hash itself, we can
 +trust that the tree is immutable and its contents never change.
 +
 +So you can trust the contents of a tree to be valid, the same way you
 +can trust the contents of a blob, but you don't know where those
 +contents _came_ from.
 +
 +Side note on trees: since a "tree" object is a sorted list of
 +"filename+content", you can create a diff between two trees without
 +actually having to unpack two trees.  Just ignore all common parts,
 +and your diff will look right.  In other words, you can effectively
 +(and efficiently) tell the difference between any two random trees by
 +O(n) where "n" is the size of the difference, rather than the size of
 +the tree.
 +
 +Side note 2 on trees: since the name of a "blob" depends entirely and
 +exclusively on its contents (i.e. there are no names or permissions
 +involved), you can see trivial renames or permission changes by
 +noticing that the blob stayed the same.  However, renames with data
 +changes need a smarter "diff" implementation.
 +
 +
 +Changeset Object
 +~~~~~~~~~~~~~~~~
 +The "changeset" object is an object that introduces the notion of
 +history into the picture.  In contrast to the other objects, it
 +doesn't just describe the physical state of a tree, it describes how
 +we got there, and why.
 +
 +A "changeset" is defined by the tree-object that it results in, the
 +parent changesets (zero, one or more) that led up to that point, and a
 +comment on what happened.  Again, a changeset is not trusted per se:
 +the contents are well-defined and "safe" due to the cryptographically
 +strong signatures at all levels, but there is no reason to believe
 +that the tree is "good" or that the merge information makes sense.
 +The parents do not have to actually have any relationship with the
 +result, for example.
 +
 +Note on changesets: unlike real SCM's, changesets do not contain
 +rename information or file mode change information.  All of that is
 +implicit in the trees involved (the result tree, and the result trees
 +of the parents), and describing that makes no sense in this idiotic
 +file manager.
 +
 +Trust Object
 +~~~~~~~~~~~~
 +The notion of "trust" is really outside the scope of "git", but it's
 +worth noting a few things.  First off, since everything is hashed with
 +SHA1, you _can_ trust that an object is intact and has not been messed
 +with by external sources.  So the name of an object uniquely
 +identifies a known state - just not a state that you may want to
 +trust.
 +
 +Furthermore, since the SHA1 signature of a changeset refers to the
 +SHA1 signatures of the tree it is associated with and the signatures
 +of the parent, a single named changeset specifies uniquely a whole set
 +of history, with full contents.  You can't later fake any step of the
 +way once you have the name of a changeset.
 +
 +So to introduce some real trust in the system, the only thing you need
 +to do is to digitally sign just _one_ special note, which includes the
 +name of a top-level changeset.  Your digital signature shows others
 +that you trust that changeset, and the immutability of the history of
 +changesets tells others that they can trust the whole history.
 +
 +In other words, you can easily validate a whole archive by just
 +sending out a single email that tells the people the name (SHA1 hash)
 +of the top changeset, and digitally sign that email using something
 +like GPG/PGP.
 +
 +In particular, you can also have a separate archive of "trust points"
 +or tags, which document your (and other peoples) trust.  You may, of
 +course, archive these "certificates of trust" using "git" itself, but
 +it's not something "git" does for you.
 +
 +Another way of saying the last point: "git" itself only handles
 +content integrity, the trust has to come from outside.
 +
 +
 +
 +
 +The "index" aka "Current Directory Cache"
 +-----------------------------------------
 The index is a simple binary file, which contains an efficient
 representation of a virtual directory content at some random time.  It
 does so by a simple array that associates a set of names, dates,
 permissions and content (aka "blob") objects together.  The cache is
 always kept ordered by name, and names are unique (with a few very
 specific rules) at any point in time, but the cache has no long-term
 -meaning, and can be partially updated at any time. 
 +meaning, and can be partially updated at any time.
 
 In particular, the index certainly does not need to be consistent with
 the current directory contents (in fact, most operations will depend on
 different ways to make the index _not_ be consistent with the directory
 hierarchy), but it has three very important attributes:
 
 - (a) it can re-generate the full state it caches (not just the directory
 -     structure: it contains pointers to the "blob" objects so that it
 -     can regenerate the data too)
 +'(a) it can re-generate the full state it caches (not just the
 +directory structure: it contains pointers to the "blob" objects so
 +that it can regenerate the data too)'
 
 -     As a special case, there is a clear and unambiguous one-way mapping
 -     from a current directory cache to a "tree object", which can be
 -     efficiently created from just the current directory cache without
 -     actually looking at any other data.  So a directory cache at any
 -     one time uniquely specifies one and only one "tree" object (but
 -     has additional data to make it easy to match up that tree object
 -     with what has happened in the directory)
 +As a special case, there is a clear and unambiguous one-way mapping
 +from a current directory cache to a "tree object", which can be
 +efficiently created from just the current directory cache without
 +actually looking at any other data.  So a directory cache at any one
 +time uniquely specifies one and only one "tree" object (but has
 +additional data to make it easy to match up that tree object with what
 +has happened in the directory)
 
 - (b) it has efficient methods for finding inconsistencies between that
 -     cached state ("tree object waiting to be instantiated") and the
 -     current state. 
 +'(b) it has efficient methods for finding inconsistencies between that
 +cached state ("tree object waiting to be instantiated") and the
 +current state.'
 
 - (c) it can additionally efficiently represent information about merge
 -     conflicts between different tree objects, allowing each pathname to
 -     be associated with sufficient information about the trees involved
 -     that you can create a three-way merge between them.
 +'(c) it can additionally efficiently represent information about merge
 +conflicts between different tree objects, allowing each pathname to be
 +associated with sufficient information about the trees involved that
 +you can create a three-way merge between them.'
 
 Those are the three ONLY things that the directory cache does.  It's a
 cache, and the normal operation is to re-generate it completely from a
 known tree object, or update/compare it with a live tree that is being
 developed.  If you blow the directory cache away entirely, you generally
 haven't lost any information as long as you have the name of the tree
 that it described. 
 
 At the same time, the directory index is at the same time also the
 staging area for creating new trees, and creating a new tree always
 involves a controlled modification of the index file.  In particular,
 the index file can have the representation of an intermediate tree that
 has not yet been instantiated.  So the index can be thought of as a
 write-back cache, which can contain dirty information that has not yet
 -been written back to the backing store. 
 +been written back to the backing store.
 
 
 
 -	The Workflow
 -
 -
 +The Workflow
 +------------
 Generally, all "git" operations work on the index file. Some operations
 -work _purely_ on the index file (showing the current state of the
 +work *purely* on the index file (showing the current state of the
 index), but most operations move data to and from the index file. Either
 from the database or from the working directory. Thus there are four
 main combinations: 
 
 - 1) working directory -> index
 +1) working directory -> index
 +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 -	You update the index with information from the working directory
 -	with the "update-cache" command.  You generally update the index
 -	information by just specifying the filename you want to update,
 -	like so:
 +You update the index with information from the working directory with
 +the "update-cache" command.  You generally update the index
 +information by just specifying the filename you want to update, like
 +so:
 
 		update-cache filename
 
 -	but to avoid common mistakes with filename globbing etc, the
 -	command will not normally add totally new entries or remove old
 -	entries, i.e. it will normally just update existing cache entries.
 +but to avoid common mistakes with filename globbing etc, the command
 +will not normally add totally new entries or remove old entries,
 +i.e. it will normally just update existing cache entries.
 
 -	To tell git that yes, you really do realize that certain files
 -	no longer exist in the archive, or that new files should be
 -	added, you should use the "--remove" and "--add" flags
 -	respectively.
 +To tell git that yes, you really do realize that certain files no
 +longer exist in the archive, or that new files should be added, you
 +should use the "--remove" and "--add" flags respectively.
 
 -	NOTE! A "--remove" flag does _not_ mean that subsequent
 -	filenames will necessarily be removed: if the files still exist
 -	in your directory structure, the index will be updated with
 -	their new status, not removed. The only thing "--remove" means
 -	is that update-cache will be considering a removed file to be a
 -	valid thing, and if the file really does not exist any more, it
 -	will update the index accordingly. 
 +NOTE! A "--remove" flag does _not_ mean that subsequent filenames will
 +necessarily be removed: if the files still exist in your directory
 +structure, the index will be updated with their new status, not
 +removed. The only thing "--remove" means is that update-cache will be
 +considering a removed file to be a valid thing, and if the file really
 +does not exist any more, it will update the index accordingly.
 
 -	As a special case, you can also do "update-cache --refresh",
 -	which will refresh the "stat" information of each index to match
 -	the current stat information. It will _not_ update the object
 -	status itself, and it will only update the fields that are used
 -	to quickly test whether an object still matches its old backing
 -	store object.
 +As a special case, you can also do "update-cache --refresh", which
 +will refresh the "stat" information of each index to match the current
 +stat information. It will _not_ update the object status itself, and
 +it will only update the fields that are used to quickly test whether
 +an object still matches its old backing store object.
 
 - 2) index -> object database
 +2) index -> object database
 +~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 -	You write your current index file to a "tree" object with the
 -	program
 +You write your current index file to a "tree" object with the program
 
 		write-tree
 
 -	that doesn't come with any options - it will just write out the
 -	current index into the set of tree objects that describe that
 -	state, and it will return the name of the resulting top-level
 -	tree. You can use that tree to re-generate the index at any time
 -	by going in the other direction:
 +that doesn't come with any options - it will just write out the
 +current index into the set of tree objects that describe that state,
 +and it will return the name of the resulting top-level tree. You can
 +use that tree to re-generate the index at any time by going in the
 +other direction:
 
 - 3) object database -> index
 +3) object database -> index
 +~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 -	You read a "tree" file from the object database, and use that to
 -	populate (and overwrite - don't do this if your index contains
 -	any unsaved state that you might want to restore later!) your
 -	current index.  Normal operation is just
 +You read a "tree" file from the object database, and use that to
 +populate (and overwrite - don't do this if your index contains any
 +unsaved state that you might want to restore later!) your current
 +index.  Normal operation is just
 
 		read-tree <sha1 of tree>
 
 -	and your index file will now be equivalent to the tree that you
 -	saved earlier. However, that is only your _index_ file: your
 -	working directory contents have not been modified.
 +and your index file will now be equivalent to the tree that you saved
 +earlier. However, that is only your _index_ file: your working
 +directory contents have not been modified.
 
 - 4) index -> working directory
 +4) index -> working directory
 +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 -	You update your working directory from the index by "checking
 -	out" files. This is not a very common operation, since normally
 -	you'd just keep your files updated, and rather than write to
 -	your working directory, you'd tell the index files about the
 -	changes in your working directory (i.e. "update-cache").
 +You update your working directory from the index by "checking out"
 +files. This is not a very common operation, since normally you'd just
 +keep your files updated, and rather than write to your working
 +directory, you'd tell the index files about the changes in your
 +working directory (i.e. "update-cache").
 
 -	However, if you decide to jump to a new version, or check out
 -	somebody else's version, or just restore a previous tree, you'd
 -	populate your index file with read-tree, and then you need to
 -	check out the result with
 +However, if you decide to jump to a new version, or check out somebody
 +else's version, or just restore a previous tree, you'd populate your
 +index file with read-tree, and then you need to check out the result
 +with
 
 		checkout-cache filename
 
 -	or, if you want to check out all of the index, use "-a".
 +or, if you want to check out all of the index, use "-a".
 
 -	NOTE! checkout-cache normally refuses to overwrite old files, so
 -	if you have an old version of the tree already checked out, you
 -	will need to use the "-f" flag (_before_ the "-a" flag or the
 -	filename) to _force_ the checkout.
 +NOTE! checkout-cache normally refuses to overwrite old files, so if
 +you have an old version of the tree already checked out, you will need
 +to use the "-f" flag (_before_ the "-a" flag or the filename) to
 +_force_ the checkout.
 
 
 -Finally, there are a few odds and ends which are not purely moving from
 -one representation to the other:
 +Finally, there are a few odds and ends which are not purely moving
 +from one representation to the other:
 
 - 5) Tying it all together
 +5) Tying it all together
 +~~~~~~~~~~~~~~~~~~~~~~~~
 
 -	To commit a tree you have instantiated with "write-tree", you'd
 -	create a "commit" object that refers to that tree and the
 -	history behind it - most notably the "parent" commits that
 -	preceded it in history. 
 +To commit a tree you have instantiated with "write-tree", you'd create
 +a "commit" object that refers to that tree and the history behind it -
 +most notably the "parent" commits that preceded it in history.
 
 -	Normally a "commit" has one parent: the previous state of the
 -	tree before a certain change was made. However, sometimes it can
 -	have two or more parent commits, in which case we call it a
 -	"merge", due to the fact that such a commit brings together
 -	("merges") two or more previous states represented by other
 -	commits. 
 +Normally a "commit" has one parent: the previous state of the tree
 +before a certain change was made. However, sometimes it can have two
 +or more parent commits, in which case we call it a "merge", due to the
 +fact that such a commit brings together ("merges") two or more
 +previous states represented by other commits.
 
 -	In other words, while a "tree" represents a particular directory
 -	state of a working directory, a "commit" represents that state
 -	in "time", and explains how we got there. 
 +In other words, while a "tree" represents a particular directory state
 +of a working directory, a "commit" represents that state in "time",
 +and explains how we got there.
 
 -	You create a commit object by giving it the tree that describes
 -	the state at the time of the commit, and a list of parents:
 +You create a commit object by giving it the tree that describes the
 +state at the time of the commit, and a list of parents:
 
 		commit-tree <tree> -p <parent> [-p <parent2> ..]
 
 -	and then giving the reason for the commit on stdin (either
 -	through redirection from a pipe or file, or by just typing it at
 -	the tty). 
 +and then giving the reason for the commit on stdin (either through
 +redirection from a pipe or file, or by just typing it at the tty).
 
 -	commit-tree will return the name of the object that represents
 -	that commit, and you should save it away for later use.
 -	Normally, you'd commit a new "HEAD" state, and while git doesn't
 -	care where you save the note about that state, in practice we
 -	tend to just write the result to the file ".git/HEAD", so that
 -	we can always see what the last committed state was.
 +commit-tree will return the name of the object that represents that
 +commit, and you should save it away for later use. Normally, you'd
 +commit a new "HEAD" state, and while git doesn't care where you save
 +the note about that state, in practice we tend to just write the
 +result to the file ".git/HEAD", so that we can always see what the
 +last committed state was.
 
 - 6) Examining the data
 +6) Examining the data
 +~~~~~~~~~~~~~~~~~~~~~
 
 -	You can examine the data represented in the object database and
 -	the index with various helper tools. For every object, you can
 -	use "cat-file" to examine details about the object:
 +You can examine the data represented in the object database and the
 +index with various helper tools. For every object, you can use
 +"cat-file" to examine details about the object:
 
 		cat-file -t <objectname>
 
 -	shows the type of the object, and once you have the type (which
 -	is usually implicit in where you find the object), you can use
 +shows the type of the object, and once you have the type (which is
 +usually implicit in where you find the object), you can use
 
 		cat-file blob|tree|commit <objectname>
 
 -	to show its contents. NOTE! Trees have binary content, and as a
 -	result there is a special helper for showing that content,
 -	called "ls-tree", which turns the binary content into a more
 -	easily readable form.
 +to show its contents. NOTE! Trees have binary content, and as a result
 +there is a special helper for showing that content, called "ls-tree",
 +which turns the binary content into a more easily readable form.
 
 -	It's especially instructive to look at "commit" objects, since
 -	those tend to be small and fairly self-explanatory. In
 -	particular, if you follow the convention of having the top
 -	commit name in ".git/HEAD", you can do
 +It's especially instructive to look at "commit" objects, since those
 +tend to be small and fairly self-explanatory. In particular, if you
 +follow the convention of having the top commit name in ".git/HEAD",
 +you can do
 
 		cat-file commit $(cat .git/HEAD)
 
 -	to see what the top commit was.
 +to see what the top commit was.
 
 - 7) Merging multiple trees
 +7) Merging multiple trees
 +~~~~~~~~~~~~~~~~~~~~~~~~~
 
 -	Git helps you do a three-way merge, which you can expand to
 -	n-way by repeating the merge procedure arbitrary times until you
 -	finally "commit" the state.  The normal situation is that you'd
 -	only do one three-way merge (two parents), and commit it, but if
 -	you like to, you can do multiple parents in one go.
 +Git helps you do a three-way merge, which you can expand to n-way by
 +repeating the merge procedure arbitrary times until you finally
 +"commit" the state.  The normal situation is that you'd only do one
 +three-way merge (two parents), and commit it, but if you like to, you
 +can do multiple parents in one go.
 
 -	To do a three-way merge, you need the two sets of "commit"
 -	objects that you want to merge, use those to find the closest
 -	common parent (a third "commit" object), and then use those
 -	commit objects to find the state of the directory ("tree"
 -	object) at these points. 
 +To do a three-way merge, you need the two sets of "commit" objects
 +that you want to merge, use those to find the closest common parent (a
 +third "commit" object), and then use those commit objects to find the
 +state of the directory ("tree" object) at these points.
 
 -	To get the "base" for the merge, you first look up the common
 -	parent of two commits with
 +To get the "base" for the merge, you first look up the common parent
 +of two commits with
 
 		merge-base <commit1> <commit2>
 
 -	which will return you the commit they are both based on.  You
 -	should now look up the "tree" objects of those commits, which
 -	you can easily do with (for example)
 +which will return you the commit they are both based on.  You should
 +now look up the "tree" objects of those commits, which you can easily
 +do with (for example)
 
 		cat-file commit <commitname> | head -1
 
 -	since the tree object information is always the first line in a
 -	commit object. 
 +since the tree object information is always the first line in a commit
 +object.
 +
 +Once you know the three trees you are going to merge (the one
 +"original" tree, aka the common case, and the two "result" trees, aka
 +the branches you want to merge), you do a "merge" read into the
 +index. This will throw away your old index contents, so you should
 +make sure that you've committed those - in fact you would normally
 +always do a merge against your last commit (which should thus match
 +what you have in your current index anyway).
 
 -	Once you know the three trees you are going to merge (the one
 -	"original" tree, aka the common case, and the two "result" trees,
 -	aka the branches you want to merge), you do a "merge" read into
 -	the index. This will throw away your old index contents, so you
 -	should make sure that you've committed those - in fact you would
 -	normally always do a merge against your last commit (which
 -	should thus match what you have in your current index anyway).
 -	To do the merge, do
 +To do the merge, do
 
 		read-tree -m <origtree> <target1tree> <target2tree>
 
 -	which will do all trivial merge operations for you directly in
 -	the index file, and you can just write the result out with
 -	"write-tree". 
 +which will do all trivial merge operations for you directly in the
 +index file, and you can just write the result out with "write-tree".
 
 -	NOTE! Because the merge is done in the index file, and not in
 -	your working directory, your working directory will no longer
 -	match your index. You can use "checkout-cache -f -a" to make the
 -	effect of the merge be seen in your working directory.
 +NOTE! Because the merge is done in the index file, and not in your
 +working directory, your working directory will no longer match your
 +index. You can use "checkout-cache -f -a" to make the effect of the
 +merge be seen in your working directory.
 
 -	NOTE2! Sadly, many merges aren't trivial. If there are files
 -	that have been added.moved or removed, or if both branches have
 -	modified the same file, you will be left with an index tree that
 -	contains "merge entries" in it. Such an index tree can _NOT_ be
 -	written out to a tree object, and you will have to resolve any
 -	such merge clashes using other tools before you can write out
 -	the result.
 +NOTE2! Sadly, many merges aren't trivial. If there are files that have
 +been added.moved or removed, or if both branches have modified the
 +same file, you will be left with an index tree that contains "merge
 +entries" in it. Such an index tree can _NOT_ be written out to a tree
 +object, and you will have to resolve any such merge clashes using
 +other tools before you can write out the result.
 
 -	[ fixme: talk about resolving merges here ]
 
 +[ fixme: talk about resolving merges here ]

 commit f7ec43ae2257241ef76a69d9d3df031a181f6ebb
 Author: Petr Baudis <[email protected]>
 Date:   Wed May 11 02:15:14 2005 +0200

    More README spelling fixes
    
    Two other README spelling fixes. I wasn't able to pinpoint the relevant
    commit in the cogito branch, but they are fairly trivial anyway.

 diff --git a/README b/README
 --- a/README
 +++ b/README
 @@ -18,443 +18,443 @@
 This is a stupid (but extremely fast) directory content manager.  It
 doesn't do a whole lot, but what it _does_ do is track directory
 contents efficiently. 
 
 There are two object abstractions: the "object database", and the
 "current directory cache" aka "index".
 
 
 
 	The Object Database (GIT_OBJECT_DIRECTORY)
 
 
 The object database is literally just a content-addressable collection
 of objects.  All objects are named by their content, which is
 approximated by the SHA1 hash of the object itself.  Objects may refer
 to other objects (by referencing their SHA1 hash), and so you can build
 up a hierarchy of objects. 
 
 All objects have a statically determined "type" aka "tag", which is
 determined at object creation time, and which identifies the format of
 the object (i.e. how it is used, and how it can refer to other objects). 
 There are currently three different object types: "blob", "tree" and
 "commit". 
 
 A "blob" object cannot refer to any other object, and is, like the tag
 implies, a pure storage object containing some user data.  It is used to
 actually store the file data, i.e. a blob object is associated with some
 particular version of some file. 
 
 A "tree" object is an object that ties one or more "blob" objects into a
 directory structure. In addition, a tree object can refer to other tree
 objects, thus creating a directory hierarchy. 
 
 Finally, a "commit" object ties such directory hierarchies together into
 a DAG of revisions - each "commit" is associated with exactly one tree
 (the directory hierarchy at the time of the commit). In addition, a
 "commit" refers to one or more "parent" commit objects that describe the
 history of how we arrived at that directory hierarchy.
 
 As a special case, a commit object with no parents is called the "root"
 object, and is the point of an initial project commit.  Each project
 must have at least one root, and while you can tie several different
 root objects together into one project by creating a commit object which
 has two or more separate roots as its ultimate parents, that's probably
 just going to confuse people.  So aim for the notion of "one root object
 per project", even if git itself does not enforce that. 
 
 Regardless of object type, all objects are share the following
 characteristics: they are all in deflated with zlib, and have a header
 that not only specifies their tag, but also size information about the
 data in the object.  It's worth noting that the SHA1 hash that is used
 to name the object is always the hash of this _compressed_ object, not
 the original data.
 
 As a result, the general consistency of an object can always be tested
 independently of the contents or the type of the object: all objects can
 be validated by verifying that (a) their hashes match the content of the
 file and (b) the object successfully inflates to a stream of bytes that
 forms a sequence of <ascii tag without space> + <space> + <ascii decimal
 size> + <byte\0> + <binary object data>. 
 
 The structured objects can further have their structure and connectivity
 to other objects verified. This is generally done with the "fsck-cache"
 program, which generates a full dependency graph of all objects, and
 verifies their internal consistency (in addition to just verifying their
 superficial consistency through the hash).
 
 The object types in some more detail:
 
   BLOB: A "blob" object is nothing but a binary blob of data, and
 	doesn't refer to anything else.  There is no signature or any
 	other verification of the data, so while the object is
 	consistent (it _is_ indexed by its sha1 hash, so the data itself
 	is certainly correct), it has absolutely no other attributes. 
 	No name associations, no permissions.  It is purely a blob of
 	data (i.e. normally "file contents"). 
 
 	In particular, since the blob is entirely defined by its data,
 	if two files in a directory tree (or in multiple different
 	versions of the repository) have the same contents, they will
 	share the same blob object. The object is totally independent
 	of it's location in the directory tree, and renaming a file does
 	not change the object that file is associated with in any way.
 
   TREE: The next hierarchical object type is the "tree" object.  A tree
 	object is a list of mode/name/blob data, sorted by name. 
 	Alternatively, the mode data may specify a directory mode, in
 	which case instead of naming a blob, that name is associated
 	with another TREE object. 
 
 	Like the "blob" object, a tree object is uniquely determined by
 	the set contents, and so two separate but identical trees will
 	always share the exact same object. This is true at all levels,
 	i.e. it's true for a "leaf" tree (which does not refer to any
 	other trees, only blobs) as well as for a whole subdirectory.
 
 	For that reason a "tree" object is just a pure data abstraction:
 	it has no history, no signatures, no verification of validity,
 	except that since the contents are again protected by the hash
 	itself, we can trust that the tree is immutable and its contents
 	never change. 
 
 	So you can trust the contents of a tree to be valid, the same
 	way you can trust the contents of a blob, but you don't know
 	where those contents _came_ from.
 
 	Side note on trees: since a "tree" object is a sorted list of
 	"filename+content", you can create a diff between two trees
 	without actually having to unpack two trees.  Just ignore all
 	common parts, and your diff will look right.  In other words,
 	you can effectively (and efficiently) tell the difference
 	between any two random trees by O(n) where "n" is the size of
 	the difference, rather than the size of the tree. 
 
 	Side note 2 on trees: since the name of a "blob" depends
 	entirely and exclusively on its contents (i.e. there are no names
 	or permissions involved), you can see trivial renames or
 	permission changes by noticing that the blob stayed the same. 
 	However, renames with data changes need a smarter "diff" implementation. 
 
 CHANGESET: The "changeset" object is an object that introduces the
 	notion of history into the picture.  In contrast to the other
 	objects, it doesn't just describe the physical state of a tree,
 	it describes how we got there, and why. 
 
 	A "changeset" is defined by the tree-object that it results in,
 	the parent changesets (zero, one or more) that led up to that
 	point, and a comment on what happened.  Again, a changeset is
 	not trusted per se: the contents are well-defined and "safe" due
 	to the cryptographically strong signatures at all levels, but
 	there is no reason to believe that the tree is "good" or that
 	the merge information makes sense.  The parents do not have to
 	actually have any relationship with the result, for example. 
 
 	Note on changesets: unlike real SCM's, changesets do not contain
 	rename information or file mode change information.  All of that
 	is implicit in the trees involved (the result tree, and the
 	result trees of the parents), and describing that makes no sense
 	in this idiotic file manager. 
 
 TRUST: The notion of "trust" is really outside the scope of "git", but
 	it's worth noting a few things.  First off, since everything is
 	hashed with SHA1, you _can_ trust that an object is intact and
 	has not been messed with by external sources.  So the name of an
 	object uniquely identifies a known state - just not a state that
 	you may want to trust. 
 
 	Furthermore, since the SHA1 signature of a changeset refers to
 	the SHA1 signatures of the tree it is associated with and the
 	signatures of the parent, a single named changeset specifies
 	uniquely a whole set of history, with full contents.  You can't
 	later fake any step of the way once you have the name of a
 	changeset. 
 
 	So to introduce some real trust in the system, the only thing
 	you need to do is to digitally sign just _one_ special note,
 	which includes the name of a top-level changeset.  Your digital
 	signature shows others that you trust that changeset, and the
 	immutability of the history of changesets tells others that they
 	can trust the whole history. 
 
 	In other words, you can easily validate a whole archive by just
 	sending out a single email that tells the people the name (SHA1
 	hash) of the top changeset, and digitally sign that email using
 	something like GPG/PGP. 
 
 	In particular, you can also have a separate archive of "trust
 	points" or tags, which document your (and other peoples) trust. 
 	You may, of course, archive these "certificates of trust" using
 	"git" itself, but it's not something "git" does for you. 
 
 Another way of saying the last point: "git" itself only handles content
 integrity, the trust has to come from outside. 
 
 
 
 	The "index" aka "Current Directory Cache" (".git/index")
 
 
 The index is a simple binary file, which contains an efficient
 representation of a virtual directory content at some random time.  It
 does so by a simple array that associates a set of names, dates,
 permissions and content (aka "blob") objects together.  The cache is
 always kept ordered by name, and names are unique (with a few very
 specific rules) at any point in time, but the cache has no long-term
 meaning, and can be partially updated at any time. 
 
 In particular, the index certainly does not need to be consistent with
 the current directory contents (in fact, most operations will depend on
 different ways to make the index _not_ be consistent with the directory
 hierarchy), but it has three very important attributes:
 
  (a) it can re-generate the full state it caches (not just the directory
      structure: it contains pointers to the "blob" objects so that it
      can regenerate the data too)
 
      As a special case, there is a clear and unambiguous one-way mapping
      from a current directory cache to a "tree object", which can be
      efficiently created from just the current directory cache without
      actually looking at any other data.  So a directory cache at any
      one time uniquely specifies one and only one "tree" object (but
      has additional data to make it easy to match up that tree object
      with what has happened in the directory)
 
  (b) it has efficient methods for finding inconsistencies between that
      cached state ("tree object waiting to be instantiated") and the
      current state. 
 
  (c) it can additionally efficiently represent information about merge
      conflicts between different tree objects, allowing each pathname to
      be associated with sufficient information about the trees involved
      that you can create a three-way merge between them.
 
 Those are the three ONLY things that the directory cache does.  It's a
 cache, and the normal operation is to re-generate it completely from a
 known tree object, or update/compare it with a live tree that is being
 developed.  If you blow the directory cache away entirely, you generally
 haven't lost any information as long as you have the name of the tree
 that it described. 
 
 At the same time, the directory index is at the same time also the
 staging area for creating new trees, and creating a new tree always
 involves a controlled modification of the index file.  In particular,
 the index file can have the representation of an intermediate tree that
 has not yet been instantiated.  So the index can be thought of as a
 write-back cache, which can contain dirty information that has not yet
 been written back to the backing store. 
 
 
 
 	The Workflow
 
 
 Generally, all "git" operations work on the index file. Some operations
 work _purely_ on the index file (showing the current state of the
 index), but most operations move data to and from the index file. Either
 from the database or from the working directory. Thus there are four
 main combinations: 
 
  1) working directory -> index
 
 	You update the index with information from the working directory
 	with the "update-cache" command.  You generally update the index
 	information by just specifying the filename you want to update,
 	like so:
 
 		update-cache filename
 
 	but to avoid common mistakes with filename globbing etc, the
 	command will not normally add totally new entries or remove old
 -	entries, i.e. it will normally just update existing cache entryes.
 +	entries, i.e. it will normally just update existing cache entries.
 
 	To tell git that yes, you really do realize that certain files
 	no longer exist in the archive, or that new files should be
 	added, you should use the "--remove" and "--add" flags
 	respectively.
 
 	NOTE! A "--remove" flag does _not_ mean that subsequent
 	filenames will necessarily be removed: if the files still exist
 	in your directory structure, the index will be updated with
 	their new status, not removed. The only thing "--remove" means
 	is that update-cache will be considering a removed file to be a
 	valid thing, and if the file really does not exist any more, it
 	will update the index accordingly. 
 
 	As a special case, you can also do "update-cache --refresh",
 	which will refresh the "stat" information of each index to match
 	the current stat information. It will _not_ update the object
 -	status itself, and it wil only update the fields that are used
 +	status itself, and it will only update the fields that are used
 	to quickly test whether an object still matches its old backing
 	store object.
 
  2) index -> object database
 
 	You write your current index file to a "tree" object with the
 	program
 
 		write-tree
 
 	that doesn't come with any options - it will just write out the
 	current index into the set of tree objects that describe that
 	state, and it will return the name of the resulting top-level
 	tree. You can use that tree to re-generate the index at any time
 	by going in the other direction:
 
  3) object database -> index
 
 	You read a "tree" file from the object database, and use that to
 	populate (and overwrite - don't do this if your index contains
 	any unsaved state that you might want to restore later!) your
 	current index.  Normal operation is just
 
 		read-tree <sha1 of tree>
 
 	and your index file will now be equivalent to the tree that you
 	saved earlier. However, that is only your _index_ file: your
 	working directory contents have not been modified.
 
  4) index -> working directory
 
 	You update your working directory from the index by "checking
 	out" files. This is not a very common operation, since normally
 	you'd just keep your files updated, and rather than write to
 	your working directory, you'd tell the index files about the
 	changes in your working directory (i.e. "update-cache").
 
 	However, if you decide to jump to a new version, or check out
 	somebody else's version, or just restore a previous tree, you'd
 	populate your index file with read-tree, and then you need to
 	check out the result with
 
 		checkout-cache filename
 
 	or, if you want to check out all of the index, use "-a".
 
 	NOTE! checkout-cache normally refuses to overwrite old files, so
 	if you have an old version of the tree already checked out, you
 	will need to use the "-f" flag (_before_ the "-a" flag or the
 	filename) to _force_ the checkout.
 
 
 Finally, there are a few odds and ends which are not purely moving from
 one representation to the other:
 
  5) Tying it all together
 
 	To commit a tree you have instantiated with "write-tree", you'd
 	create a "commit" object that refers to that tree and the
 	history behind it - most notably the "parent" commits that
 	preceded it in history. 
 
 	Normally a "commit" has one parent: the previous state of the
 	tree before a certain change was made. However, sometimes it can
 	have two or more parent commits, in which case we call it a
 	"merge", due to the fact that such a commit brings together
 	("merges") two or more previous states represented by other
 	commits. 
 
 	In other words, while a "tree" represents a particular directory
 	state of a working directory, a "commit" represents that state
 	in "time", and explains how we got there. 
 
 	You create a commit object by giving it the tree that describes
 	the state at the time of the commit, and a list of parents:
 
 		commit-tree <tree> -p <parent> [-p <parent2> ..]
 
 	and then giving the reason for the commit on stdin (either
 	through redirection from a pipe or file, or by just typing it at
 	the tty). 
 
 	commit-tree will return the name of the object that represents
 	that commit, and you should save it away for later use.
 	Normally, you'd commit a new "HEAD" state, and while git doesn't
 	care where you save the note about that state, in practice we
 	tend to just write the result to the file ".git/HEAD", so that
 	we can always see what the last committed state was.
 
  6) Examining the data
 
 	You can examine the data represented in the object database and
 	the index with various helper tools. For every object, you can
 	use "cat-file" to examine details about the object:
 
 		cat-file -t <objectname>
 
 	shows the type of the object, and once you have the type (which
 	is usually implicit in where you find the object), you can use
 
 		cat-file blob|tree|commit <objectname>
 
 	to show its contents. NOTE! Trees have binary content, and as a
 	result there is a special helper for showing that content,
 	called "ls-tree", which turns the binary content into a more
 	easily readable form.
 
 	It's especially instructive to look at "commit" objects, since
 	those tend to be small and fairly self-explanatory. In
 	particular, if you follow the convention of having the top
 	commit name in ".git/HEAD", you can do
 
 		cat-file commit $(cat .git/HEAD)
 
 	to see what the top commit was.
 
  7) Merging multiple trees
 
 	Git helps you do a three-way merge, which you can expand to
 	n-way by repeating the merge procedure arbitrary times until you
 	finally "commit" the state.  The normal situation is that you'd
 	only do one three-way merge (two parents), and commit it, but if
 	you like to, you can do multiple parents in one go.
 
 	To do a three-way merge, you need the two sets of "commit"
 	objects that you want to merge, use those to find the closest
 	common parent (a third "commit" object), and then use those
 	commit objects to find the state of the directory ("tree"
 	object) at these points. 
 
 	To get the "base" for the merge, you first look up the common
 	parent of two commits with
 
 		merge-base <commit1> <commit2>
 
 	which will return you the commit they are both based on.  You
 	should now look up the "tree" objects of those commits, which
 	you can easily do with (for example)
 
 		cat-file commit <commitname> | head -1
 
 	since the tree object information is always the first line in a
 	commit object. 
 
 	Once you know the three trees you are going to merge (the one
 	"original" tree, aka the common case, and the two "result" trees,
 	aka the branches you want to merge), you do a "merge" read into
 	the index. This will throw away your old index contents, so you
 	should make sure that you've committed those - in fact you would
 	normally always do a merge against your last commit (which
 	should thus match what you have in your current index anyway).
 	To do the merge, do
 
 		read-tree -m <origtree> <target1tree> <target2tree>
 
 	which will do all trivial merge operations for you directly in
 	the index file, and you can just write the result out with
 	"write-tree". 
 
 	NOTE! Because the merge is done in the index file, and not in
 	your working directory, your working directory will no longer
 	match your index. You can use "checkout-cache -f -a" to make the
 	effect of the merge be seen in your working directory.
 
 	NOTE2! Sadly, many merges aren't trivial. If there are files
 	that have been added.moved or removed, or if both branches have
 	modified the same file, you will be left with an index tree that
 	contains "merge entries" in it. Such an index tree can _NOT_ be
 	written out to a tree object, and you will have to resolve any
 	such merge clashes using other tools before you can write out
 	the result.
 
 	[ fixme: talk about resolving merges here ]
 

 commit bebc5c61439a291e8f0f99e037ebdf63d3dfac46
 Author: Zack Brown <[email protected]>
 Date:   Mon Apr 25 14:28:33 2005 -0700

    Spelling fixes in README.
    
    Signed-off-by: Zack Brown <[email protected]>
    Signed-off-by: Petr Baudis <[email protected]>

 diff --git a/README b/README
 --- a/README
 +++ b/README
 @@ -18,443 +18,443 @@
 This is a stupid (but extremely fast) directory content manager.  It
 doesn't do a whole lot, but what it _does_ do is track directory
 contents efficiently. 
 
 There are two object abstractions: the "object database", and the
 "current directory cache" aka "index".
 
 
 
 	The Object Database (GIT_OBJECT_DIRECTORY)
 
 
 The object database is literally just a content-addressable collection
 of objects.  All objects are named by their content, which is
 approximated by the SHA1 hash of the object itself.  Objects may refer
 to other objects (by referencing their SHA1 hash), and so you can build
 up a hierarchy of objects. 
 
 All objects have a statically determined "type" aka "tag", which is
 determined at object creation time, and which identifies the format of
 the object (i.e. how it is used, and how it can refer to other objects). 
 There are currently three different object types: "blob", "tree" and
 "commit". 
 
 A "blob" object cannot refer to any other object, and is, like the tag
 implies, a pure storage object containing some user data.  It is used to
 actually store the file data, i.e. a blob object is associated with some
 particular version of some file. 
 
 A "tree" object is an object that ties one or more "blob" objects into a
 directory structure. In addition, a tree object can refer to other tree
 objects, thus creating a directory hierarchy. 
 
 Finally, a "commit" object ties such directory hierarchies together into
 a DAG of revisions - each "commit" is associated with exactly one tree
 (the directory hierarchy at the time of the commit). In addition, a
 "commit" refers to one or more "parent" commit objects that describe the
 history of how we arrived at that directory hierarchy.
 
 As a special case, a commit object with no parents is called the "root"
 object, and is the point of an initial project commit.  Each project
 must have at least one root, and while you can tie several different
 root objects together into one project by creating a commit object which
 has two or more separate roots as its ultimate parents, that's probably
 just going to confuse people.  So aim for the notion of "one root object
 per project", even if git itself does not enforce that. 
 
 Regardless of object type, all objects are share the following
 characteristics: they are all in deflated with zlib, and have a header
 that not only specifies their tag, but also size information about the
 data in the object.  It's worth noting that the SHA1 hash that is used
 to name the object is always the hash of this _compressed_ object, not
 the original data.
 
 As a result, the general consistency of an object can always be tested
 independently of the contents or the type of the object: all objects can
 be validated by verifying that (a) their hashes match the content of the
 file and (b) the object successfully inflates to a stream of bytes that
 forms a sequence of <ascii tag without space> + <space> + <ascii decimal
 size> + <byte\0> + <binary object data>. 
 
 The structured objects can further have their structure and connectivity
 to other objects verified. This is generally done with the "fsck-cache"
 program, which generates a full dependency graph of all objects, and
 verifies their internal consistency (in addition to just verifying their
 superficial consistency through the hash).
 
 The object types in some more detail:
 
   BLOB: A "blob" object is nothing but a binary blob of data, and
 	doesn't refer to anything else.  There is no signature or any
 	other verification of the data, so while the object is
 	consistent (it _is_ indexed by its sha1 hash, so the data itself
 	is certainly correct), it has absolutely no other attributes. 
 	No name associations, no permissions.  It is purely a blob of
 	data (i.e. normally "file contents"). 
 
 	In particular, since the blob is entirely defined by its data,
 	if two files in a directory tree (or in multiple different
 	versions of the repository) have the same contents, they will
 -	share the same blob object. The object is toally independent
 +	share the same blob object. The object is totally independent
 	of it's location in the directory tree, and renaming a file does
 	not change the object that file is associated with in any way.
 
   TREE: The next hierarchical object type is the "tree" object.  A tree
 	object is a list of mode/name/blob data, sorted by name. 
 	Alternatively, the mode data may specify a directory mode, in
 	which case instead of naming a blob, that name is associated
 	with another TREE object. 
 
 	Like the "blob" object, a tree object is uniquely determined by
 	the set contents, and so two separate but identical trees will
 	always share the exact same object. This is true at all levels,
 	i.e. it's true for a "leaf" tree (which does not refer to any
 	other trees, only blobs) as well as for a whole subdirectory.
 
 	For that reason a "tree" object is just a pure data abstraction:
 	it has no history, no signatures, no verification of validity,
 	except that since the contents are again protected by the hash
 	itself, we can trust that the tree is immutable and its contents
 	never change. 
 
 	So you can trust the contents of a tree to be valid, the same
 	way you can trust the contents of a blob, but you don't know
 	where those contents _came_ from.
 
 	Side note on trees: since a "tree" object is a sorted list of
 	"filename+content", you can create a diff between two trees
 	without actually having to unpack two trees.  Just ignore all
 	common parts, and your diff will look right.  In other words,
 	you can effectively (and efficiently) tell the difference
 	between any two random trees by O(n) where "n" is the size of
 	the difference, rather than the size of the tree. 
 
 	Side note 2 on trees: since the name of a "blob" depends
 	entirely and exclusively on its contents (i.e. there are no names
 	or permissions involved), you can see trivial renames or
 	permission changes by noticing that the blob stayed the same. 
 	However, renames with data changes need a smarter "diff" implementation. 
 
 CHANGESET: The "changeset" object is an object that introduces the
 	notion of history into the picture.  In contrast to the other
 	objects, it doesn't just describe the physical state of a tree,
 	it describes how we got there, and why. 
 
 	A "changeset" is defined by the tree-object that it results in,
 	the parent changesets (zero, one or more) that led up to that
 	point, and a comment on what happened.  Again, a changeset is
 	not trusted per se: the contents are well-defined and "safe" due
 	to the cryptographically strong signatures at all levels, but
 	there is no reason to believe that the tree is "good" or that
 	the merge information makes sense.  The parents do not have to
 	actually have any relationship with the result, for example. 
 
 	Note on changesets: unlike real SCM's, changesets do not contain
 	rename information or file mode change information.  All of that
 	is implicit in the trees involved (the result tree, and the
 	result trees of the parents), and describing that makes no sense
 	in this idiotic file manager. 
 
 TRUST: The notion of "trust" is really outside the scope of "git", but
 	it's worth noting a few things.  First off, since everything is
 	hashed with SHA1, you _can_ trust that an object is intact and
 	has not been messed with by external sources.  So the name of an
 	object uniquely identifies a known state - just not a state that
 	you may want to trust. 
 
 	Furthermore, since the SHA1 signature of a changeset refers to
 	the SHA1 signatures of the tree it is associated with and the
 	signatures of the parent, a single named changeset specifies
 	uniquely a whole set of history, with full contents.  You can't
 	later fake any step of the way once you have the name of a
 	changeset. 
 
 	So to introduce some real trust in the system, the only thing
 	you need to do is to digitally sign just _one_ special note,
 	which includes the name of a top-level changeset.  Your digital
 	signature shows others that you trust that changeset, and the
 	immutability of the history of changesets tells others that they
 	can trust the whole history. 
 
 	In other words, you can easily validate a whole archive by just
 	sending out a single email that tells the people the name (SHA1
 	hash) of the top changeset, and digitally sign that email using
 	something like GPG/PGP. 
 
 	In particular, you can also have a separate archive of "trust
 	points" or tags, which document your (and other peoples) trust. 
 	You may, of course, archive these "certificates of trust" using
 	"git" itself, but it's not something "git" does for you. 
 
 Another way of saying the last point: "git" itself only handles content
 integrity, the trust has to come from outside. 
 
 
 
 	The "index" aka "Current Directory Cache" (".git/index")
 
 
 The index is a simple binary file, which contains an efficient
 representation of a virtual directory content at some random time.  It
 does so by a simple array that associates a set of names, dates,
 permissions and content (aka "blob") objects together.  The cache is
 always kept ordered by name, and names are unique (with a few very
 specific rules) at any point in time, but the cache has no long-term
 meaning, and can be partially updated at any time. 
 
 In particular, the index certainly does not need to be consistent with
 the current directory contents (in fact, most operations will depend on
 different ways to make the index _not_ be consistent with the directory
 hierarchy), but it has three very important attributes:
 
  (a) it can re-generate the full state it caches (not just the directory
      structure: it contains pointers to the "blob" objects so that it
      can regenerate the data too)
 
      As a special case, there is a clear and unambiguous one-way mapping
      from a current directory cache to a "tree object", which can be
      efficiently created from just the current directory cache without
      actually looking at any other data.  So a directory cache at any
      one time uniquely specifies one and only one "tree" object (but
      has additional data to make it easy to match up that tree object
      with what has happened in the directory)
 
  (b) it has efficient methods for finding inconsistencies between that
      cached state ("tree object waiting to be instantiated") and the
      current state. 
 
  (c) it can additionally efficiently represent information about merge
      conflicts between different tree objects, allowing each pathname to
      be associated with sufficient information about the trees involved
      that you can create a three-way merge between them.
 
 Those are the three ONLY things that the directory cache does.  It's a
 cache, and the normal operation is to re-generate it completely from a
 known tree object, or update/compare it with a live tree that is being
 developed.  If you blow the directory cache away entirely, you generally
 haven't lost any information as long as you have the name of the tree
 that it described. 
 
 At the same time, the directory index is at the same time also the
 staging area for creating new trees, and creating a new tree always
 involves a controlled modification of the index file.  In particular,
 the index file can have the representation of an intermediate tree that
 has not yet been instantiated.  So the index can be thought of as a
 write-back cache, which can contain dirty information that has not yet
 been written back to the backing store. 
 
 
 
 	The Workflow
 
 
 Generally, all "git" operations work on the index file. Some operations
 work _purely_ on the index file (showing the current state of the
 index), but most operations move data to and from the index file. Either
 from the database or from the working directory. Thus there are four
 main combinations: 
 
  1) working directory -> index
 
 	You update the index with information from the working directory
 	with the "update-cache" command.  You generally update the index
 	information by just specifying the filename you want to update,
 	like so:
 
 		update-cache filename
 
 	but to avoid common mistakes with filename globbing etc, the
 	command will not normally add totally new entries or remove old
 	entries, i.e. it will normally just update existing cache entryes.
 
 	To tell git that yes, you really do realize that certain files
 	no longer exist in the archive, or that new files should be
 	added, you should use the "--remove" and "--add" flags
 	respectively.
 
 	NOTE! A "--remove" flag does _not_ mean that subsequent
 	filenames will necessarily be removed: if the files still exist
 	in your directory structure, the index will be updated with
 	their new status, not removed. The only thing "--remove" means
 	is that update-cache will be considering a removed file to be a
 	valid thing, and if the file really does not exist any more, it
 	will update the index accordingly. 
 
 	As a special case, you can also do "update-cache --refresh",
 	which will refresh the "stat" information of each index to match
 	the current stat information. It will _not_ update the object
 	status itself, and it wil only update the fields that are used
 	to quickly test whether an object still matches its old backing
 	store object.
 
  2) index -> object database
 
 	You write your current index file to a "tree" object with the
 	program
 
 		write-tree
 
 	that doesn't come with any options - it will just write out the
 	current index into the set of tree objects that describe that
 	state, and it will return the name of the resulting top-level
 	tree. You can use that tree to re-generate the index at any time
 	by going in the other direction:
 
  3) object database -> index
 
 	You read a "tree" file from the object database, and use that to
 	populate (and overwrite - don't do this if your index contains
 	any unsaved state that you might want to restore later!) your
 	current index.  Normal operation is just
 
 		read-tree <sha1 of tree>
 
 	and your index file will now be equivalent to the tree that you
 	saved earlier. However, that is only your _index_ file: your
 	working directory contents have not been modified.
 
  4) index -> working directory
 
 	You update your working directory from the index by "checking
 	out" files. This is not a very common operation, since normally
 	you'd just keep your files updated, and rather than write to
 	your working directory, you'd tell the index files about the
 	changes in your working directory (i.e. "update-cache").
 
 	However, if you decide to jump to a new version, or check out
 -	somebody elses version, or just restore a previous tree, you'd
 +	somebody else's version, or just restore a previous tree, you'd
 	populate your index file with read-tree, and then you need to
 	check out the result with
 
 		checkout-cache filename
 
 	or, if you want to check out all of the index, use "-a".
 
 	NOTE! checkout-cache normally refuses to overwrite old files, so
 	if you have an old version of the tree already checked out, you
 	will need to use the "-f" flag (_before_ the "-a" flag or the
 	filename) to _force_ the checkout.
 
 
 Finally, there are a few odds and ends which are not purely moving from
 one representation to the other:
 
  5) Tying it all together
 
 	To commit a tree you have instantiated with "write-tree", you'd
 	create a "commit" object that refers to that tree and the
 	history behind it - most notably the "parent" commits that
 	preceded it in history. 
 
 	Normally a "commit" has one parent: the previous state of the
 	tree before a certain change was made. However, sometimes it can
 	have two or more parent commits, in which case we call it a
 	"merge", due to the fact that such a commit brings together
 	("merges") two or more previous states represented by other
 	commits. 
 
 	In other words, while a "tree" represents a particular directory
 	state of a working directory, a "commit" represents that state
 	in "time", and explains how we got there. 
 
 	You create a commit object by giving it the tree that describes
 	the state at the time of the commit, and a list of parents:
 
 		commit-tree <tree> -p <parent> [-p <parent2> ..]
 
 	and then giving the reason for the commit on stdin (either
 	through redirection from a pipe or file, or by just typing it at
 	the tty). 
 
 	commit-tree will return the name of the object that represents
 	that commit, and you should save it away for later use.
 	Normally, you'd commit a new "HEAD" state, and while git doesn't
 	care where you save the note about that state, in practice we
 	tend to just write the result to the file ".git/HEAD", so that
 	we can always see what the last committed state was.
 
  6) Examining the data
 
 	You can examine the data represented in the object database and
 	the index with various helper tools. For every object, you can
 	use "cat-file" to examine details about the object:
 
 		cat-file -t <objectname>
 
 	shows the type of the object, and once you have the type (which
 	is usually implicit in where you find the object), you can use
 
 		cat-file blob|tree|commit <objectname>
 
 	to show its contents. NOTE! Trees have binary content, and as a
 	result there is a special helper for showing that content,
 	called "ls-tree", which turns the binary content into a more
 	easily readable form.
 
 	It's especially instructive to look at "commit" objects, since
 	those tend to be small and fairly self-explanatory. In
 	particular, if you follow the convention of having the top
 	commit name in ".git/HEAD", you can do
 
 		cat-file commit $(cat .git/HEAD)
 
 	to see what the top commit was.
 
  7) Merging multiple trees
 
 	Git helps you do a three-way merge, which you can expand to
 	n-way by repeating the merge procedure arbitrary times until you
 	finally "commit" the state.  The normal situation is that you'd
 	only do one three-way merge (two parents), and commit it, but if
 	you like to, you can do multiple parents in one go.
 
 	To do a three-way merge, you need the two sets of "commit"
 	objects that you want to merge, use those to find the closest
 	common parent (a third "commit" object), and then use those
 	commit objects to find the state of the directory ("tree"
 	object) at these points. 
 
 	To get the "base" for the merge, you first look up the common
 	parent of two commits with
 
 		merge-base <commit1> <commit2>
 
 	which will return you the commit they are both based on.  You
 	should now look up the "tree" objects of those commits, which
 	you can easily do with (for example)
 
 		cat-file commit <commitname> | head -1
 
 	since the tree object information is always the first line in a
 	commit object. 
 
 	Once you know the three trees you are going to merge (the one
 	"original" tree, aka the common case, and the two "result" trees,
 	aka the branches you want to merge), you do a "merge" read into
 	the index. This will throw away your old index contents, so you
 	should make sure that you've committed those - in fact you would
 	normally always do a merge against your last commit (which
 	should thus match what you have in your current index anyway).
 	To do the merge, do
 
 		read-tree -m <origtree> <target1tree> <target2tree>
 
 	which will do all trivial merge operations for you directly in
 	the index file, and you can just write the result out with
 	"write-tree". 
 
 	NOTE! Because the merge is done in the index file, and not in
 	your working directory, your working directory will no longer
 	match your index. You can use "checkout-cache -f -a" to make the
 	effect of the merge be seen in your working directory.
 
 	NOTE2! Sadly, many merges aren't trivial. If there are files
 	that have been added.moved or removed, or if both branches have
 	modified the same file, you will be left with an index tree that
 	contains "merge entries" in it. Such an index tree can _NOT_ be
 	written out to a tree object, and you will have to resolve any
 	such merge clashes using other tools before you can write out
 	the result.
 
 	[ fixme: talk about resolving merges here ]
 

 commit 90c4851b1787dfc1d3d3a5cf78c558d136c69bdd
 Author: Pavel Roskin <[email protected]>
 Date:   Thu Apr 14 23:35:00 2005 -0400

    [PATCH] Misc fixes for git-pasky
    
    * README: spell checked
    
    Signed-off-by: Pavel Roskin <[email protected]>
    
    Few more s/ie/i.e./ fixes.
    
    Signed-off-by: Petr Baudis <[email protected]>

 diff --git a/README b/README
 --- a/README
 +++ b/README
 @@ -18,443 +18,443 @@
 This is a stupid (but extremely fast) directory content manager.  It
 doesn't do a whole lot, but what it _does_ do is track directory
 contents efficiently. 
 
 There are two object abstractions: the "object database", and the
 "current directory cache" aka "index".
 
 
 
 	The Object Database (GIT_OBJECT_DIRECTORY)
 
 
 The object database is literally just a content-addressable collection
 of objects.  All objects are named by their content, which is
 approximated by the SHA1 hash of the object itself.  Objects may refer
 to other objects (by referencing their SHA1 hash), and so you can build
 up a hierarchy of objects. 
 
 All objects have a statically determined "type" aka "tag", which is
 determined at object creation time, and which identifies the format of
 -the object (ie how it is used, and how it can refer to other objects). 
 +the object (i.e. how it is used, and how it can refer to other objects). 
 There are currently three different object types: "blob", "tree" and
 "commit". 
 
 A "blob" object cannot refer to any other object, and is, like the tag
 implies, a pure storage object containing some user data.  It is used to
 -actually store the file data, ie a blob object is associated with some
 +actually store the file data, i.e. a blob object is associated with some
 particular version of some file. 
 
 A "tree" object is an object that ties one or more "blob" objects into a
 directory structure. In addition, a tree object can refer to other tree
 objects, thus creating a directory hierarchy. 
 
 Finally, a "commit" object ties such directory hierarchies together into
 a DAG of revisions - each "commit" is associated with exactly one tree
 (the directory hierarchy at the time of the commit). In addition, a
 "commit" refers to one or more "parent" commit objects that describe the
 history of how we arrived at that directory hierarchy.
 
 As a special case, a commit object with no parents is called the "root"
 object, and is the point of an initial project commit.  Each project
 must have at least one root, and while you can tie several different
 root objects together into one project by creating a commit object which
 has two or more separate roots as its ultimate parents, that's probably
 just going to confuse people.  So aim for the notion of "one root object
 per project", even if git itself does not enforce that. 
 
 Regardless of object type, all objects are share the following
 characteristics: they are all in deflated with zlib, and have a header
 that not only specifies their tag, but also size information about the
 data in the object.  It's worth noting that the SHA1 hash that is used
 to name the object is always the hash of this _compressed_ object, not
 the original data.
 
 As a result, the general consistency of an object can always be tested
 independently of the contents or the type of the object: all objects can
 be validated by verifying that (a) their hashes match the content of the
 file and (b) the object successfully inflates to a stream of bytes that
 forms a sequence of <ascii tag without space> + <space> + <ascii decimal
 size> + <byte\0> + <binary object data>. 
 
 The structured objects can further have their structure and connectivity
 to other objects verified. This is generally done with the "fsck-cache"
 program, which generates a full dependency graph of all objects, and
 verifies their internal consistency (in addition to just verifying their
 superficial consistency through the hash).
 
 The object types in some more detail:
 
   BLOB: A "blob" object is nothing but a binary blob of data, and
 	doesn't refer to anything else.  There is no signature or any
 	other verification of the data, so while the object is
 	consistent (it _is_ indexed by its sha1 hash, so the data itself
 	is certainly correct), it has absolutely no other attributes. 
 	No name associations, no permissions.  It is purely a blob of
 -	data (ie normally "file contents"). 
 +	data (i.e. normally "file contents"). 
 
 	In particular, since the blob is entirely defined by its data,
 	if two files in a directory tree (or in multiple different
 	versions of the repository) have the same contents, they will
 	share the same blob object. The object is toally independent
 	of it's location in the directory tree, and renaming a file does
 	not change the object that file is associated with in any way.
 
   TREE: The next hierarchical object type is the "tree" object.  A tree
 	object is a list of mode/name/blob data, sorted by name. 
 	Alternatively, the mode data may specify a directory mode, in
 	which case instead of naming a blob, that name is associated
 	with another TREE object. 
 
 	Like the "blob" object, a tree object is uniquely determined by
 	the set contents, and so two separate but identical trees will
 	always share the exact same object. This is true at all levels,
 -	ie it's true for a "leaf" tree (which does not refer to any
 +	i.e. it's true for a "leaf" tree (which does not refer to any
 	other trees, only blobs) as well as for a whole subdirectory.
 
 	For that reason a "tree" object is just a pure data abstraction:
 	it has no history, no signatures, no verification of validity,
 	except that since the contents are again protected by the hash
 	itself, we can trust that the tree is immutable and its contents
 	never change. 
 
 	So you can trust the contents of a tree to be valid, the same
 	way you can trust the contents of a blob, but you don't know
 	where those contents _came_ from.
 
 	Side note on trees: since a "tree" object is a sorted list of
 	"filename+content", you can create a diff between two trees
 	without actually having to unpack two trees.  Just ignore all
 	common parts, and your diff will look right.  In other words,
 	you can effectively (and efficiently) tell the difference
 	between any two random trees by O(n) where "n" is the size of
 	the difference, rather than the size of the tree. 
 
 	Side note 2 on trees: since the name of a "blob" depends
 -	entirely and exclusively on its contents (ie there are no names
 +	entirely and exclusively on its contents (i.e. there are no names
 	or permissions involved), you can see trivial renames or
 	permission changes by noticing that the blob stayed the same. 
 	However, renames with data changes need a smarter "diff" implementation. 
 
 CHANGESET: The "changeset" object is an object that introduces the
 	notion of history into the picture.  In contrast to the other
 	objects, it doesn't just describe the physical state of a tree,
 	it describes how we got there, and why. 
 
 	A "changeset" is defined by the tree-object that it results in,
 	the parent changesets (zero, one or more) that led up to that
 	point, and a comment on what happened.  Again, a changeset is
 	not trusted per se: the contents are well-defined and "safe" due
 	to the cryptographically strong signatures at all levels, but
 	there is no reason to believe that the tree is "good" or that
 	the merge information makes sense.  The parents do not have to
 	actually have any relationship with the result, for example. 
 
 	Note on changesets: unlike real SCM's, changesets do not contain
 -	rename information or file mode chane information.  All of that
 +	rename information or file mode change information.  All of that
 	is implicit in the trees involved (the result tree, and the
 	result trees of the parents), and describing that makes no sense
 	in this idiotic file manager. 
 
 TRUST: The notion of "trust" is really outside the scope of "git", but
 	it's worth noting a few things.  First off, since everything is
 	hashed with SHA1, you _can_ trust that an object is intact and
 	has not been messed with by external sources.  So the name of an
 	object uniquely identifies a known state - just not a state that
 	you may want to trust. 
 
 	Furthermore, since the SHA1 signature of a changeset refers to
 	the SHA1 signatures of the tree it is associated with and the
 	signatures of the parent, a single named changeset specifies
 	uniquely a whole set of history, with full contents.  You can't
 	later fake any step of the way once you have the name of a
 	changeset. 
 
 	So to introduce some real trust in the system, the only thing
 	you need to do is to digitally sign just _one_ special note,
 	which includes the name of a top-level changeset.  Your digital
 	signature shows others that you trust that changeset, and the
 	immutability of the history of changesets tells others that they
 	can trust the whole history. 
 
 	In other words, you can easily validate a whole archive by just
 	sending out a single email that tells the people the name (SHA1
 	hash) of the top changeset, and digitally sign that email using
 	something like GPG/PGP. 
 
 	In particular, you can also have a separate archive of "trust
 	points" or tags, which document your (and other peoples) trust. 
 	You may, of course, archive these "certificates of trust" using
 	"git" itself, but it's not something "git" does for you. 
 
 Another way of saying the last point: "git" itself only handles content
 integrity, the trust has to come from outside. 
 
 
 
 	The "index" aka "Current Directory Cache" (".git/index")
 
 
 The index is a simple binary file, which contains an efficient
 representation of a virtual directory content at some random time.  It
 does so by a simple array that associates a set of names, dates,
 permissions and content (aka "blob") objects together.  The cache is
 always kept ordered by name, and names are unique (with a few very
 specific rules) at any point in time, but the cache has no long-term
 meaning, and can be partially updated at any time. 
 
 In particular, the index certainly does not need to be consistent with
 the current directory contents (in fact, most operations will depend on
 different ways to make the index _not_ be consistent with the directory
 hierarchy), but it has three very important attributes:
 
  (a) it can re-generate the full state it caches (not just the directory
      structure: it contains pointers to the "blob" objects so that it
      can regenerate the data too)
 
      As a special case, there is a clear and unambiguous one-way mapping
      from a current directory cache to a "tree object", which can be
      efficiently created from just the current directory cache without
      actually looking at any other data.  So a directory cache at any
      one time uniquely specifies one and only one "tree" object (but
      has additional data to make it easy to match up that tree object
      with what has happened in the directory)
 
  (b) it has efficient methods for finding inconsistencies between that
      cached state ("tree object waiting to be instantiated") and the
      current state. 
 
  (c) it can additionally efficiently represent information about merge
      conflicts between different tree objects, allowing each pathname to
      be associated with sufficient information about the trees involved
      that you can create a three-way merge between them.
 
 Those are the three ONLY things that the directory cache does.  It's a
 cache, and the normal operation is to re-generate it completely from a
 known tree object, or update/compare it with a live tree that is being
 developed.  If you blow the directory cache away entirely, you generally
 haven't lost any information as long as you have the name of the tree
 that it described. 
 
 At the same time, the directory index is at the same time also the
 staging area for creating new trees, and creating a new tree always
 involves a controlled modification of the index file.  In particular,
 the index file can have the representation of an intermediate tree that
 has not yet been instantiated.  So the index can be thought of as a
 write-back cache, which can contain dirty information that has not yet
 been written back to the backing store. 
 
 
 
 	The Workflow
 
 
 Generally, all "git" operations work on the index file. Some operations
 work _purely_ on the index file (showing the current state of the
 index), but most operations move data to and from the index file. Either
 from the database or from the working directory. Thus there are four
 main combinations: 
 
  1) working directory -> index
 
 	You update the index with information from the working directory
 	with the "update-cache" command.  You generally update the index
 	information by just specifying the filename you want to update,
 	like so:
 
 		update-cache filename
 
 	but to avoid common mistakes with filename globbing etc, the
 	command will not normally add totally new entries or remove old
 -	entries, ie it will normally just update existing cache entryes.
 +	entries, i.e. it will normally just update existing cache entryes.
 
 	To tell git that yes, you really do realize that certain files
 	no longer exist in the archive, or that new files should be
 	added, you should use the "--remove" and "--add" flags
 	respectively.
 
 	NOTE! A "--remove" flag does _not_ mean that subsequent
 	filenames will necessarily be removed: if the files still exist
 	in your directory structure, the index will be updated with
 	their new status, not removed. The only thing "--remove" means
 	is that update-cache will be considering a removed file to be a
 	valid thing, and if the file really does not exist any more, it
 	will update the index accordingly. 
 
 	As a special case, you can also do "update-cache --refresh",
 	which will refresh the "stat" information of each index to match
 	the current stat information. It will _not_ update the object
 	status itself, and it wil only update the fields that are used
 	to quickly test whether an object still matches its old backing
 	store object.
 
  2) index -> object database
 
 	You write your current index file to a "tree" object with the
 	program
 
 		write-tree
 
 	that doesn't come with any options - it will just write out the
 	current index into the set of tree objects that describe that
 	state, and it will return the name of the resulting top-level
 	tree. You can use that tree to re-generate the index at any time
 	by going in the other direction:
 
  3) object database -> index
 
 	You read a "tree" file from the object database, and use that to
 	populate (and overwrite - don't do this if your index contains
 	any unsaved state that you might want to restore later!) your
 	current index.  Normal operation is just
 
 		read-tree <sha1 of tree>
 
 	and your index file will now be equivalent to the tree that you
 	saved earlier. However, that is only your _index_ file: your
 	working directory contents have not been modified.
 
  4) index -> working directory
 
 	You update your working directory from the index by "checking
 	out" files. This is not a very common operation, since normally
 	you'd just keep your files updated, and rather than write to
 	your working directory, you'd tell the index files about the
 -	changes in your working directory (ie "update-cache").
 +	changes in your working directory (i.e. "update-cache").
 
 	However, if you decide to jump to a new version, or check out
 	somebody elses version, or just restore a previous tree, you'd
 	populate your index file with read-tree, and then you need to
 	check out the result with
 
 		checkout-cache filename
 
 	or, if you want to check out all of the index, use "-a".
 
 	NOTE! checkout-cache normally refuses to overwrite old files, so
 	if you have an old version of the tree already checked out, you
 	will need to use the "-f" flag (_before_ the "-a" flag or the
 	filename) to _force_ the checkout.
 
 
 Finally, there are a few odds and ends which are not purely moving from
 one representation to the other:
 
  5) Tying it all together
 
 	To commit a tree you have instantiated with "write-tree", you'd
 	create a "commit" object that refers to that tree and the
 	history behind it - most notably the "parent" commits that
 	preceded it in history. 
 
 	Normally a "commit" has one parent: the previous state of the
 	tree before a certain change was made. However, sometimes it can
 	have two or more parent commits, in which case we call it a
 	"merge", due to the fact that such a commit brings together
 	("merges") two or more previous states represented by other
 	commits. 
 
 	In other words, while a "tree" represents a particular directory
 	state of a working directory, a "commit" represents that state
 	in "time", and explains how we got there. 
 
 	You create a commit object by giving it the tree that describes
 	the state at the time of the commit, and a list of parents:
 
 		commit-tree <tree> -p <parent> [-p <parent2> ..]
 
 	and then giving the reason for the commit on stdin (either
 	through redirection from a pipe or file, or by just typing it at
 	the tty). 
 
 	commit-tree will return the name of the object that represents
 	that commit, and you should save it away for later use.
 	Normally, you'd commit a new "HEAD" state, and while git doesn't
 	care where you save the note about that state, in practice we
 	tend to just write the result to the file ".git/HEAD", so that
 	we can always see what the last committed state was.
 
  6) Examining the data
 
 	You can examine the data represented in the object database and
 	the index with various helper tools. For every object, you can
 	use "cat-file" to examine details about the object:
 
 		cat-file -t <objectname>
 
 	shows the type of the object, and once you have the type (which
 	is usually implicit in where you find the object), you can use
 
 		cat-file blob|tree|commit <objectname>
 
 	to show its contents. NOTE! Trees have binary content, and as a
 	result there is a special helper for showing that content,
 	called "ls-tree", which turns the binary content into a more
 	easily readable form.
 
 	It's especially instructive to look at "commit" objects, since
 	those tend to be small and fairly self-explanatory. In
 	particular, if you follow the convention of having the top
 	commit name in ".git/HEAD", you can do
 
 		cat-file commit $(cat .git/HEAD)
 
 	to see what the top commit was.
 
  7) Merging multiple trees
 
 	Git helps you do a three-way merge, which you can expand to
 	n-way by repeating the merge procedure arbitrary times until you
 	finally "commit" the state.  The normal situation is that you'd
 	only do one three-way merge (two parents), and commit it, but if
 	you like to, you can do multiple parents in one go.
 
 	To do a three-way merge, you need the two sets of "commit"
 	objects that you want to merge, use those to find the closest
 	common parent (a third "commit" object), and then use those
 	commit objects to find the state of the directory ("tree"
 	object) at these points. 
 
 	To get the "base" for the merge, you first look up the common
 	parent of two commits with
 
 		merge-base <commit1> <commit2>
 
 	which will return you the commit they are both based on.  You
 	should now look up the "tree" objects of those commits, which
 	you can easily do with (for example)
 
 		cat-file commit <commitname> | head -1
 
 	since the tree object information is always the first line in a
 	commit object. 
 
 	Once you know the three trees you are going to merge (the one
 	"original" tree, aka the common case, and the two "result" trees,
 	aka the branches you want to merge), you do a "merge" read into
 	the index. This will throw away your old index contents, so you
 	should make sure that you've committed those - in fact you would
 	normally always do a merge against your last commit (which
 	should thus match what you have in your current index anyway).
 	To do the merge, do
 
 		read-tree -m <origtree> <target1tree> <target2tree>
 
 	which will do all trivial merge operations for you directly in
 	the index file, and you can just write the result out with
 	"write-tree". 
 
 	NOTE! Because the merge is done in the index file, and not in
 	your working directory, your working directory will no longer
 	match your index. You can use "checkout-cache -f -a" to make the
 	effect of the merge be seen in your working directory.
 
 	NOTE2! Sadly, many merges aren't trivial. If there are files
 	that have been added.moved or removed, or if both branches have
 	modified the same file, you will be left with an index tree that
 	contains "merge entries" in it. Such an index tree can _NOT_ be
 	written out to a tree object, and you will have to resolve any
 	such merge clashes using other tools before you can write out
 	the result.
 
 	[ fixme: talk about resolving merges here ]
 

 commit d19938ab6053e3dad75a68a60ef8cad1f378b0e5
 Author: Junio C Hamano <[email protected]>
 Date:   Mon May 9 17:57:56 2005 -0700

    Rename environment variables.
    
    H. Peter Anvin mentioned that using SHA1_whatever as an
    environment variable name is not nice and we should instead use
    names starting with "GIT_" prefix to avoid conflicts.  Here is
    what this patch does:
    
     * Renames the following environment variables:
    
        New name                           Old Name
    
        GIT_AUTHOR_DATE                    AUTHOR_DATE
        GIT_AUTHOR_EMAIL                   AUTHOR_EMAIL
        GIT_AUTHOR_NAME                    AUTHOR_NAME
        GIT_COMMITTER_EMAIL                COMMIT_AUTHOR_EMAIL
        GIT_COMMITTER_NAME                 COMMIT_AUTHOR_NAME
        GIT_ALTERNATE_OBJECT_DIRECTORIES   SHA1_FILE_DIRECTORIES
        GIT_OBJECT_DIRECTORY               SHA1_FILE_DIRECTORY
    
     * Introduces a compatibility macro, gitenv(), which does an
       getenv() and if it fails calls gitenv_bc(), which in turn
       picks up the value from old name while giving a warning about
       using an old name.
    
     * Changes all users of the environment variable to fetch
       environment variable with the new name using gitenv().
    
     * Updates the documentation and scripts shipped with Linus GIT
       distribution.
    
    The transition plan is as follows:
    
     * We will keep the backward compatibility list used by gitenv()
       for now, so the current scripts and user environments
       continue to work as before.  The users will get warnings when
       they have old name but not new name in their environment to
       the stderr.
    
     * The Porcelain layers should start using new names.  However,
       just in case it ends up calling old Plumbing layer
       implementation, they should also export old names, taking
       values from the corresponding new names, during the
       transition period.
    
     * After a transition period, we would drop the compatibility
       support and drop gitenv().  Revert the callers to directly
       call getenv() but keep using the new names.
    
       The last part is probably optional and the transition
       duration needs to be set to a reasonable value.
    
    Signed-off-by: Junio C Hamano <[email protected]>

 diff --git a/README b/README
 --- a/README
 +++ b/README
 @@ -18,443 +18,443 @@
 This is a stupid (but extremely fast) directory content manager.  It
 doesn't do a whole lot, but what it _does_ do is track directory
 contents efficiently. 
 
 There are two object abstractions: the "object database", and the
 "current directory cache" aka "index".
 
 
 
 -	The Object Database (SHA1_FILE_DIRECTORY)
 +	The Object Database (GIT_OBJECT_DIRECTORY)
 
 
 The object database is literally just a content-addressable collection
 of objects.  All objects are named by their content, which is
 approximated by the SHA1 hash of the object itself.  Objects may refer
 to other objects (by referencing their SHA1 hash), and so you can build
 up a hierarchy of objects. 
 
 All objects have a statically determined "type" aka "tag", which is
 determined at object creation time, and which identifies the format of
 the object (ie how it is used, and how it can refer to other objects). 
 There are currently three different object types: "blob", "tree" and
 "commit". 
 
 A "blob" object cannot refer to any other object, and is, like the tag
 implies, a pure storage object containing some user data.  It is used to
 actually store the file data, ie a blob object is associated with some
 particular version of some file. 
 
 A "tree" object is an object that ties one or more "blob" objects into a
 directory structure. In addition, a tree object can refer to other tree
 objects, thus creating a directory hierarchy. 
 
 Finally, a "commit" object ties such directory hierarchies together into
 a DAG of revisions - each "commit" is associated with exactly one tree
 (the directory hierarchy at the time of the commit). In addition, a
 "commit" refers to one or more "parent" commit objects that describe the
 history of how we arrived at that directory hierarchy.
 
 As a special case, a commit object with no parents is called the "root"
 object, and is the point of an initial project commit.  Each project
 must have at least one root, and while you can tie several different
 root objects together into one project by creating a commit object which
 has two or more separate roots as its ultimate parents, that's probably
 just going to confuse people.  So aim for the notion of "one root object
 per project", even if git itself does not enforce that. 
 
 Regardless of object type, all objects are share the following
 characteristics: they are all in deflated with zlib, and have a header
 that not only specifies their tag, but also size information about the
 data in the object.  It's worth noting that the SHA1 hash that is used
 to name the object is always the hash of this _compressed_ object, not
 the original data.
 
 As a result, the general consistency of an object can always be tested
 independently of the contents or the type of the object: all objects can
 be validated by verifying that (a) their hashes match the content of the
 file and (b) the object successfully inflates to a stream of bytes that
 forms a sequence of <ascii tag without space> + <space> + <ascii decimal
 size> + <byte\0> + <binary object data>. 
 
 The structured objects can further have their structure and connectivity
 to other objects verified. This is generally done with the "fsck-cache"
 program, which generates a full dependency graph of all objects, and
 verifies their internal consistency (in addition to just verifying their
 superficial consistency through the hash).
 
 The object types in some more detail:
 
   BLOB: A "blob" object is nothing but a binary blob of data, and
 	doesn't refer to anything else.  There is no signature or any
 	other verification of the data, so while the object is
 	consistent (it _is_ indexed by its sha1 hash, so the data itself
 	is certainly correct), it has absolutely no other attributes. 
 	No name associations, no permissions.  It is purely a blob of
 	data (ie normally "file contents"). 
 
 	In particular, since the blob is entirely defined by its data,
 	if two files in a directory tree (or in multiple different
 	versions of the repository) have the same contents, they will
 	share the same blob object. The object is toally independent
 	of it's location in the directory tree, and renaming a file does
 	not change the object that file is associated with in any way.
 
   TREE: The next hierarchical object type is the "tree" object.  A tree
 	object is a list of mode/name/blob data, sorted by name. 
 	Alternatively, the mode data may specify a directory mode, in
 	which case instead of naming a blob, that name is associated
 	with another TREE object. 
 
 	Like the "blob" object, a tree object is uniquely determined by
 	the set contents, and so two separate but identical trees will
 	always share the exact same object. This is true at all levels,
 	ie it's true for a "leaf" tree (which does not refer to any
 	other trees, only blobs) as well as for a whole subdirectory.
 
 	For that reason a "tree" object is just a pure data abstraction:
 	it has no history, no signatures, no verification of validity,
 	except that since the contents are again protected by the hash
 	itself, we can trust that the tree is immutable and its contents
 	never change. 
 
 	So you can trust the contents of a tree to be valid, the same
 	way you can trust the contents of a blob, but you don't know
 	where those contents _came_ from.
 
 	Side note on trees: since a "tree" object is a sorted list of
 	"filename+content", you can create a diff between two trees
 	without actually having to unpack two trees.  Just ignore all
 	common parts, and your diff will look right.  In other words,
 	you can effectively (and efficiently) tell the difference
 	between any two random trees by O(n) where "n" is the size of
 	the difference, rather than the size of the tree. 
 
 	Side note 2 on trees: since the name of a "blob" depends
 	entirely and exclusively on its contents (ie there are no names
 	or permissions involved), you can see trivial renames or
 	permission changes by noticing that the blob stayed the same. 
 	However, renames with data changes need a smarter "diff" implementation. 
 
 CHANGESET: The "changeset" object is an object that introduces the
 	notion of history into the picture.  In contrast to the other
 	objects, it doesn't just describe the physical state of a tree,
 	it describes how we got there, and why. 
 
 	A "changeset" is defined by the tree-object that it results in,
 	the parent changesets (zero, one or more) that led up to that
 	point, and a comment on what happened.  Again, a changeset is
 	not trusted per se: the contents are well-defined and "safe" due
 	to the cryptographically strong signatures at all levels, but
 	there is no reason to believe that the tree is "good" or that
 	the merge information makes sense.  The parents do not have to
 	actually have any relationship with the result, for example. 
 
 	Note on changesets: unlike real SCM's, changesets do not contain
 	rename information or file mode chane information.  All of that
 	is implicit in the trees involved (the result tree, and the
 	result trees of the parents), and describing that makes no sense
 	in this idiotic file manager. 
 
 TRUST: The notion of "trust" is really outside the scope of "git", but
 	it's worth noting a few things.  First off, since everything is
 	hashed with SHA1, you _can_ trust that an object is intact and
 	has not been messed with by external sources.  So the name of an
 	object uniquely identifies a known state - just not a state that
 	you may want to trust. 
 
 	Furthermore, since the SHA1 signature of a changeset refers to
 	the SHA1 signatures of the tree it is associated with and the
 	signatures of the parent, a single named changeset specifies
 	uniquely a whole set of history, with full contents.  You can't
 	later fake any step of the way once you have the name of a
 	changeset. 
 
 	So to introduce some real trust in the system, the only thing
 	you need to do is to digitally sign just _one_ special note,
 	which includes the name of a top-level changeset.  Your digital
 	signature shows others that you trust that changeset, and the
 	immutability of the history of changesets tells others that they
 	can trust the whole history. 
 
 	In other words, you can easily validate a whole archive by just
 	sending out a single email that tells the people the name (SHA1
 	hash) of the top changeset, and digitally sign that email using
 	something like GPG/PGP. 
 
 	In particular, you can also have a separate archive of "trust
 	points" or tags, which document your (and other peoples) trust. 
 	You may, of course, archive these "certificates of trust" using
 	"git" itself, but it's not something "git" does for you. 
 
 Another way of saying the last point: "git" itself only handles content
 integrity, the trust has to come from outside. 
 
 
 
 	The "index" aka "Current Directory Cache" (".git/index")
 
 
 The index is a simple binary file, which contains an efficient
 representation of a virtual directory content at some random time.  It
 does so by a simple array that associates a set of names, dates,
 permissions and content (aka "blob") objects together.  The cache is
 always kept ordered by name, and names are unique (with a few very
 specific rules) at any point in time, but the cache has no long-term
 meaning, and can be partially updated at any time. 
 
 In particular, the index certainly does not need to be consistent with
 the current directory contents (in fact, most operations will depend on
 different ways to make the index _not_ be consistent with the directory
 hierarchy), but it has three very important attributes:
 
  (a) it can re-generate the full state it caches (not just the directory
      structure: it contains pointers to the "blob" objects so that it
      can regenerate the data too)
 
      As a special case, there is a clear and unambiguous one-way mapping
      from a current directory cache to a "tree object", which can be
      efficiently created from just the current directory cache without
      actually looking at any other data.  So a directory cache at any
      one time uniquely specifies one and only one "tree" object (but
      has additional data to make it easy to match up that tree object
      with what has happened in the directory)
 
  (b) it has efficient methods for finding inconsistencies between that
      cached state ("tree object waiting to be instantiated") and the
      current state. 
 
  (c) it can additionally efficiently represent information about merge
      conflicts between different tree objects, allowing each pathname to
      be associated with sufficient information about the trees involved
      that you can create a three-way merge between them.
 
 Those are the three ONLY things that the directory cache does.  It's a
 cache, and the normal operation is to re-generate it completely from a
 known tree object, or update/compare it with a live tree that is being
 developed.  If you blow the directory cache away entirely, you generally
 haven't lost any information as long as you have the name of the tree
 that it described. 
 
 At the same time, the directory index is at the same time also the
 staging area for creating new trees, and creating a new tree always
 involves a controlled modification of the index file.  In particular,
 the index file can have the representation of an intermediate tree that
 has not yet been instantiated.  So the index can be thought of as a
 write-back cache, which can contain dirty information that has not yet
 been written back to the backing store. 
 
 
 
 	The Workflow
 
 
 Generally, all "git" operations work on the index file. Some operations
 work _purely_ on the index file (showing the current state of the
 index), but most operations move data to and from the index file. Either
 from the database or from the working directory. Thus there are four
 main combinations: 
 
  1) working directory -> index
 
 	You update the index with information from the working directory
 	with the "update-cache" command.  You generally update the index
 	information by just specifying the filename you want to update,
 	like so:
 
 		update-cache filename
 
 	but to avoid common mistakes with filename globbing etc, the
 	command will not normally add totally new entries or remove old
 	entries, ie it will normally just update existing cache entryes.
 
 	To tell git that yes, you really do realize that certain files
 	no longer exist in the archive, or that new files should be
 	added, you should use the "--remove" and "--add" flags
 	respectively.
 
 	NOTE! A "--remove" flag does _not_ mean that subsequent
 	filenames will necessarily be removed: if the files still exist
 	in your directory structure, the index will be updated with
 	their new status, not removed. The only thing "--remove" means
 	is that update-cache will be considering a removed file to be a
 	valid thing, and if the file really does not exist any more, it
 	will update the index accordingly. 
 
 	As a special case, you can also do "update-cache --refresh",
 	which will refresh the "stat" information of each index to match
 	the current stat information. It will _not_ update the object
 	status itself, and it wil only update the fields that are used
 	to quickly test whether an object still matches its old backing
 	store object.
 
  2) index -> object database
 
 	You write your current index file to a "tree" object with the
 	program
 
 		write-tree
 
 	that doesn't come with any options - it will just write out the
 	current index into the set of tree objects that describe that
 	state, and it will return the name of the resulting top-level
 	tree. You can use that tree to re-generate the index at any time
 	by going in the other direction:
 
  3) object database -> index
 
 	You read a "tree" file from the object database, and use that to
 	populate (and overwrite - don't do this if your index contains
 	any unsaved state that you might want to restore later!) your
 	current index.  Normal operation is just
 
 		read-tree <sha1 of tree>
 
 	and your index file will now be equivalent to the tree that you
 	saved earlier. However, that is only your _index_ file: your
 	working directory contents have not been modified.
 
  4) index -> working directory
 
 	You update your working directory from the index by "checking
 	out" files. This is not a very common operation, since normally
 	you'd just keep your files updated, and rather than write to
 	your working directory, you'd tell the index files about the
 	changes in your working directory (ie "update-cache").
 
 	However, if you decide to jump to a new version, or check out
 	somebody elses version, or just restore a previous tree, you'd
 	populate your index file with read-tree, and then you need to
 	check out the result with
 
 		checkout-cache filename
 
 	or, if you want to check out all of the index, use "-a".
 
 	NOTE! checkout-cache normally refuses to overwrite old files, so
 	if you have an old version of the tree already checked out, you
 	will need to use the "-f" flag (_before_ the "-a" flag or the
 	filename) to _force_ the checkout.
 
 
 Finally, there are a few odds and ends which are not purely moving from
 one representation to the other:
 
  5) Tying it all together
 
 	To commit a tree you have instantiated with "write-tree", you'd
 	create a "commit" object that refers to that tree and the
 	history behind it - most notably the "parent" commits that
 	preceded it in history. 
 
 	Normally a "commit" has one parent: the previous state of the
 	tree before a certain change was made. However, sometimes it can
 	have two or more parent commits, in which case we call it a
 	"merge", due to the fact that such a commit brings together
 	("merges") two or more previous states represented by other
 	commits. 
 
 	In other words, while a "tree" represents a particular directory
 	state of a working directory, a "commit" represents that state
 	in "time", and explains how we got there. 
 
 	You create a commit object by giving it the tree that describes
 	the state at the time of the commit, and a list of parents:
 
 		commit-tree <tree> -p <parent> [-p <parent2> ..]
 
 	and then giving the reason for the commit on stdin (either
 	through redirection from a pipe or file, or by just typing it at
 	the tty). 
 
 	commit-tree will return the name of the object that represents
 	that commit, and you should save it away for later use.
 	Normally, you'd commit a new "HEAD" state, and while git doesn't
 	care where you save the note about that state, in practice we
 	tend to just write the result to the file ".git/HEAD", so that
 	we can always see what the last committed state was.
 
  6) Examining the data
 
 	You can examine the data represented in the object database and
 	the index with various helper tools. For every object, you can
 	use "cat-file" to examine details about the object:
 
 		cat-file -t <objectname>
 
 	shows the type of the object, and once you have the type (which
 	is usually implicit in where you find the object), you can use
 
 		cat-file blob|tree|commit <objectname>
 
 	to show its contents. NOTE! Trees have binary content, and as a
 	result there is a special helper for showing that content,
 	called "ls-tree", which turns the binary content into a more
 	easily readable form.
 
 	It's especially instructive to look at "commit" objects, since
 	those tend to be small and fairly self-explanatory. In
 	particular, if you follow the convention of having the top
 	commit name in ".git/HEAD", you can do
 
 		cat-file commit $(cat .git/HEAD)
 
 	to see what the top commit was.
 
  7) Merging multiple trees
 
 	Git helps you do a three-way merge, which you can expand to
 	n-way by repeating the merge procedure arbitrary times until you
 	finally "commit" the state.  The normal situation is that you'd
 	only do one three-way merge (two parents), and commit it, but if
 	you like to, you can do multiple parents in one go.
 
 	To do a three-way merge, you need the two sets of "commit"
 	objects that you want to merge, use those to find the closest
 	common parent (a third "commit" object), and then use those
 	commit objects to find the state of the directory ("tree"
 	object) at these points. 
 
 	To get the "base" for the merge, you first look up the common
 	parent of two commits with
 
 		merge-base <commit1> <commit2>
 
 	which will return you the commit they are both based on.  You
 	should now look up the "tree" objects of those commits, which
 	you can easily do with (for example)
 
 		cat-file commit <commitname> | head -1
 
 	since the tree object information is always the first line in a
 	commit object. 
 
 	Once you know the three trees you are going to merge (the one
 	"original" tree, aka the common case, and the two "result" trees,
 	aka the branches you want to merge), you do a "merge" read into
 	the index. This will throw away your old index contents, so you
 	should make sure that you've committed those - in fact you would
 	normally always do a merge against your last commit (which
 	should thus match what you have in your current index anyway).
 	To do the merge, do
 
 		read-tree -m <origtree> <target1tree> <target2tree>
 
 	which will do all trivial merge operations for you directly in
 	the index file, and you can just write the result out with
 	"write-tree". 
 
 	NOTE! Because the merge is done in the index file, and not in
 	your working directory, your working directory will no longer
 	match your index. You can use "checkout-cache -f -a" to make the
 	effect of the merge be seen in your working directory.
 
 	NOTE2! Sadly, many merges aren't trivial. If there are files
 	that have been added.moved or removed, or if both branches have
 	modified the same file, you will be left with an index tree that
 	contains "merge entries" in it. Such an index tree can _NOT_ be
 	written out to a tree object, and you will have to resolve any
 	such merge clashes using other tools before you can write out
 	the result.
 
 	[ fixme: talk about resolving merges here ]
 

 commit 6ad6d3d36c5924c8ff502ebbb6a6216df01e7efb
 Author: Linus Torvalds <[email protected]>
 Date:   Sun Apr 17 21:52:23 2005 -0700

    Update README to reflect the hierarchical tree objects,
    and other newfangled things like merging.
    
    Also, talk more about the actual operations, and give some
    rough examples of what you can do.

 diff --git a/README b/README
 --- a/README
 +++ b/README
 @@ -15,154 +18,443 @@
 This is a stupid (but extremely fast) directory content manager.  It
 doesn't do a whole lot, but what it _does_ do is track directory
 contents efficiently. 
 
 There are two object abstractions: the "object database", and the
 -"current directory cache".
 +"current directory cache" aka "index".
 +
 +
 
 	The Object Database (SHA1_FILE_DIRECTORY)
 
 +
 The object database is literally just a content-addressable collection
 of objects.  All objects are named by their content, which is
 approximated by the SHA1 hash of the object itself.  Objects may refer
 to other objects (by referencing their SHA1 hash), and so you can build
 up a hierarchy of objects. 
 
 -There are several kinds of objects in the content-addressable collection
 -database.  They are all in deflated with zlib, and start off with a tag
 -of their type, and size information about the data.  The SHA1 hash is
 -always the hash of the _compressed_ object, not the original one.
 -
 -In particular, the consistency of an object can always be tested
 +All objects have a statically determined "type" aka "tag", which is
 +determined at object creation time, and which identifies the format of
 +the object (ie how it is used, and how it can refer to other objects). 
 +There are currently three different object types: "blob", "tree" and
 +"commit". 
 +
 +A "blob" object cannot refer to any other object, and is, like the tag
 +implies, a pure storage object containing some user data.  It is used to
 +actually store the file data, ie a blob object is associated with some
 +particular version of some file. 
 +
 +A "tree" object is an object that ties one or more "blob" objects into a
 +directory structure. In addition, a tree object can refer to other tree
 +objects, thus creating a directory hierarchy. 
 +
 +Finally, a "commit" object ties such directory hierarchies together into
 +a DAG of revisions - each "commit" is associated with exactly one tree
 +(the directory hierarchy at the time of the commit). In addition, a
 +"commit" refers to one or more "parent" commit objects that describe the
 +history of how we arrived at that directory hierarchy.
 +
 +As a special case, a commit object with no parents is called the "root"
 +object, and is the point of an initial project commit.  Each project
 +must have at least one root, and while you can tie several different
 +root objects together into one project by creating a commit object which
 +has two or more separate roots as its ultimate parents, that's probably
 +just going to confuse people.  So aim for the notion of "one root object
 +per project", even if git itself does not enforce that. 
 +
 +Regardless of object type, all objects are share the following
 +characteristics: they are all in deflated with zlib, and have a header
 +that not only specifies their tag, but also size information about the
 +data in the object.  It's worth noting that the SHA1 hash that is used
 +to name the object is always the hash of this _compressed_ object, not
 +the original data.
 +
 +As a result, the general consistency of an object can always be tested
 independently of the contents or the type of the object: all objects can
 be validated by verifying that (a) their hashes match the content of the
 file and (b) the object successfully inflates to a stream of bytes that
 forms a sequence of <ascii tag without space> + <space> + <ascii decimal
 size> + <byte\0> + <binary object data>. 
 
 -BLOB: A "blob" object is nothing but a binary blob of data, and doesn't
 -refer to anything else.  There is no signature or any other verification
 -of the data, so while the object is consistent (it _is_ indexed by its
 -sha1 hash, so the data itself is certainly correct), it has absolutely
 -no other attributes.  No name associations, no permissions.  It is
 -purely a blob of data (ie normally "file contents"). 
 -
 -TREE: The next hierarchical object type is the "tree" object.  A tree
 -object is a list of permission/name/blob data, sorted by name.  In other
 -words the tree object is uniquely determined by the set contents, and so
 -two separate but identical trees will always share the exact same
 -object. 
 -
 -Again, a "tree" object is just a pure data abstraction: it has no
 -history, no signatures, no verification of validity, except that the
 -contents are again protected by the hash itself.  So you can trust the
 -contents of a tree, the same way you can trust the contents of a blob,
 -but you don't know where those contents _came_ from. 
 -
 -Side note on trees: since a "tree" object is a sorted list of
 -"filename+content", you can create a diff between two trees without
 -actually having to unpack two trees.  Just ignore all common parts, and
 -your diff will look right.  In other words, you can effectively (and
 -efficiently) tell the difference between any two random trees by O(n)
 -where "n" is the size of the difference, rather than the size of the
 -tree. 
 -
 -Side note 2 on trees: since the name of a "blob" depends entirely and
 -exclusively on its contents (ie there are no names or permissions
 -involved), you can see trivial renames or permission changes by noticing
 -that the blob stayed the same.  However, renames with data changes need
 -a smarter "diff" implementation. 
 +The structured objects can further have their structure and connectivity
 +to other objects verified. This is generally done with the "fsck-cache"
 +program, which generates a full dependency graph of all objects, and
 +verifies their internal consistency (in addition to just verifying their
 +superficial consistency through the hash).
 +
 +The object types in some more detail:
 +
 +  BLOB: A "blob" object is nothing but a binary blob of data, and
 +	doesn't refer to anything else.  There is no signature or any
 +	other verification of the data, so while the object is
 +	consistent (it _is_ indexed by its sha1 hash, so the data itself
 +	is certainly correct), it has absolutely no other attributes. 
 +	No name associations, no permissions.  It is purely a blob of
 +	data (ie normally "file contents"). 
 +
 +	In particular, since the blob is entirely defined by its data,
 +	if two files in a directory tree (or in multiple different
 +	versions of the repository) have the same contents, they will
 +	share the same blob object. The object is toally independent
 +	of it's location in the directory tree, and renaming a file does
 +	not change the object that file is associated with in any way.
 +
 +  TREE: The next hierarchical object type is the "tree" object.  A tree
 +	object is a list of mode/name/blob data, sorted by name. 
 +	Alternatively, the mode data may specify a directory mode, in
 +	which case instead of naming a blob, that name is associated
 +	with another TREE object. 
 +
 +	Like the "blob" object, a tree object is uniquely determined by
 +	the set contents, and so two separate but identical trees will
 +	always share the exact same object. This is true at all levels,
 +	ie it's true for a "leaf" tree (which does not refer to any
 +	other trees, only blobs) as well as for a whole subdirectory.
 +
 +	For that reason a "tree" object is just a pure data abstraction:
 +	it has no history, no signatures, no verification of validity,
 +	except that since the contents are again protected by the hash
 +	itself, we can trust that the tree is immutable and its contents
 +	never change. 
 +
 +	So you can trust the contents of a tree to be valid, the same
 +	way you can trust the contents of a blob, but you don't know
 +	where those contents _came_ from.
 +
 +	Side note on trees: since a "tree" object is a sorted list of
 +	"filename+content", you can create a diff between two trees
 +	without actually having to unpack two trees.  Just ignore all
 +	common parts, and your diff will look right.  In other words,
 +	you can effectively (and efficiently) tell the difference
 +	between any two random trees by O(n) where "n" is the size of
 +	the difference, rather than the size of the tree. 
 +
 +	Side note 2 on trees: since the name of a "blob" depends
 +	entirely and exclusively on its contents (ie there are no names
 +	or permissions involved), you can see trivial renames or
 +	permission changes by noticing that the blob stayed the same. 
 +	However, renames with data changes need a smarter "diff" implementation. 
 
 CHANGESET: The "changeset" object is an object that introduces the
 -notion of history into the picture.  In contrast to the other objects,
 -it doesn't just describe the physical state of a tree, it describes how
 -we got there, and why. 
 -
 -A "changeset" is defined by the tree-object that it results in, the
 -parent changesets (zero, one or more) that led up to that point, and a
 -comment on what happened. Again, a changeset is not trusted per se:
 -the contents are well-defined and "safe" due to the cryptographically
 -strong signatures at all levels, but there is no reason to believe that
 -the tree is "good" or that the merge information makes sense. The
 -parents do not have to actually have any relationship with the result,
 -for example.
 -
 -Note on changesets: unlike real SCM's, changesets do not contain rename
 -information or file mode chane information.  All of that is implicit in
 -the trees involved (the result tree, and the result trees of the
 -parents), and describing that makes no sense in this idiotic file
 -manager.
 +	notion of history into the picture.  In contrast to the other
 +	objects, it doesn't just describe the physical state of a tree,
 +	it describes how we got there, and why. 
 +
 +	A "changeset" is defined by the tree-object that it results in,
 +	the parent changesets (zero, one or more) that led up to that
 +	point, and a comment on what happened.  Again, a changeset is
 +	not trusted per se: the contents are well-defined and "safe" due
 +	to the cryptographically strong signatures at all levels, but
 +	there is no reason to believe that the tree is "good" or that
 +	the merge information makes sense.  The parents do not have to
 +	actually have any relationship with the result, for example. 
 +
 +	Note on changesets: unlike real SCM's, changesets do not contain
 +	rename information or file mode chane information.  All of that
 +	is implicit in the trees involved (the result tree, and the
 +	result trees of the parents), and describing that makes no sense
 +	in this idiotic file manager. 
 
 TRUST: The notion of "trust" is really outside the scope of "git", but
 -it's worth noting a few things. First off, since everything is hashed
 -with SHA1, you _can_ trust that an object is intact and has not been
 -messed with by external sources. So the name of an object uniquely
 -identifies a known state - just not a state that you may want to trust.
 -
 -Furthermore, since the SHA1 signature of a changeset refers to the
 -SHA1 signatures of the tree it is associated with and the signatures
 -of the parent, a single named changeset specifies uniquely a whole
 -set of history, with full contents. You can't later fake any step of
 -the way once you have the name of a changeset.
 -
 -So to introduce some real trust in the system, the only thing you need
 -to do is to digitally sign just _one_ special note, which includes the
 -name of a top-level changeset.  Your digital signature shows others that
 -you trust that changeset, and the immutability of the history of
 -changesets tells others that they can trust the whole history.
 -
 -In other words, you can easily validate a whole archive by just sending
 -out a single email that tells the people the name (SHA1 hash) of the top
 -changeset, and digitally sign that email using something like GPG/PGP.
 -
 -In particular, you can also have a separate archive of "trust points" or
 -tags, which document your (and other peoples) trust.  You may, of
 -course, archive these "certificates of trust" using "git" itself, but
 -it's not something "git" does for you. 
 -
 -Another way of saying the same thing: "git" itself only handles content
 +	it's worth noting a few things.  First off, since everything is
 +	hashed with SHA1, you _can_ trust that an object is intact and
 +	has not been messed with by external sources.  So the name of an
 +	object uniquely identifies a known state - just not a state that
 +	you may want to trust. 
 +
 +	Furthermore, since the SHA1 signature of a changeset refers to
 +	the SHA1 signatures of the tree it is associated with and the
 +	signatures of the parent, a single named changeset specifies
 +	uniquely a whole set of history, with full contents.  You can't
 +	later fake any step of the way once you have the name of a
 +	changeset. 
 +
 +	So to introduce some real trust in the system, the only thing
 +	you need to do is to digitally sign just _one_ special note,
 +	which includes the name of a top-level changeset.  Your digital
 +	signature shows others that you trust that changeset, and the
 +	immutability of the history of changesets tells others that they
 +	can trust the whole history. 
 +
 +	In other words, you can easily validate a whole archive by just
 +	sending out a single email that tells the people the name (SHA1
 +	hash) of the top changeset, and digitally sign that email using
 +	something like GPG/PGP. 
 +
 +	In particular, you can also have a separate archive of "trust
 +	points" or tags, which document your (and other peoples) trust. 
 +	You may, of course, archive these "certificates of trust" using
 +	"git" itself, but it's not something "git" does for you. 
 +
 +Another way of saying the last point: "git" itself only handles content
 integrity, the trust has to come from outside. 
 
 -	Current Directory Cache (".git/index")
 
 -The "current directory cache" is a simple binary file, which contains an
 -efficient representation of a virtual directory content at some random
 -time.  It does so by a simple array that associates a set of names,
 -dates, permissions and content (aka "blob") objects together.  The cache
 -is always kept ordered by name, and names are unique at any point in
 -time, but the cache has no long-term meaning, and can be partially
 -updated at any time. 
 
 -In particular, the "current directory cache" certainly does not need to
 -be consistent with the current directory contents, but it has two very
 -important attributes:
 +	The "index" aka "Current Directory Cache" (".git/index")
 +
 +
 +The index is a simple binary file, which contains an efficient
 +representation of a virtual directory content at some random time.  It
 +does so by a simple array that associates a set of names, dates,
 +permissions and content (aka "blob") objects together.  The cache is
 +always kept ordered by name, and names are unique (with a few very
 +specific rules) at any point in time, but the cache has no long-term
 +meaning, and can be partially updated at any time. 
 +
 +In particular, the index certainly does not need to be consistent with
 +the current directory contents (in fact, most operations will depend on
 +different ways to make the index _not_ be consistent with the directory
 +hierarchy), but it has three very important attributes:
 
  (a) it can re-generate the full state it caches (not just the directory
 -     structure: through the "blob" object it can regenerate the data too)
 +     structure: it contains pointers to the "blob" objects so that it
 +     can regenerate the data too)
 
      As a special case, there is a clear and unambiguous one-way mapping
      from a current directory cache to a "tree object", which can be
      efficiently created from just the current directory cache without
      actually looking at any other data.  So a directory cache at any
      one time uniquely specifies one and only one "tree" object (but
      has additional data to make it easy to match up that tree object
      with what has happened in the directory)
 -    
 -
 -and
 
  (b) it has efficient methods for finding inconsistencies between that
      cached state ("tree object waiting to be instantiated") and the
      current state. 
 
 -Those are the two ONLY things that the directory cache does.  It's a
 + (c) it can additionally efficiently represent information about merge
 +     conflicts between different tree objects, allowing each pathname to
 +     be associated with sufficient information about the trees involved
 +     that you can create a three-way merge between them.
 +
 +Those are the three ONLY things that the directory cache does.  It's a
 cache, and the normal operation is to re-generate it completely from a
 known tree object, or update/compare it with a live tree that is being
 -developed.  If you blow the directory cache away entirely, you haven't
 -lost any information as long as you have the name of the tree that it
 -described. 
 -
 -(But directory caches can also have real information in them: in
 -particular, they can have the representation of an intermediate tree
 -that has not yet been instantiated.  So they do have meaning and usage
 -outside of caching - in one sense you can think of the current directory
 -cache as being the "work in progress" towards a tree commit).
 +developed.  If you blow the directory cache away entirely, you generally
 +haven't lost any information as long as you have the name of the tree
 +that it described. 
 +
 +At the same time, the directory index is at the same time also the
 +staging area for creating new trees, and creating a new tree always
 +involves a controlled modification of the index file.  In particular,
 +the index file can have the representation of an intermediate tree that
 +has not yet been instantiated.  So the index can be thought of as a
 +write-back cache, which can contain dirty information that has not yet
 +been written back to the backing store. 
 +
 +
 +
 +	The Workflow
 +
 +
 +Generally, all "git" operations work on the index file. Some operations
 +work _purely_ on the index file (showing the current state of the
 +index), but most operations move data to and from the index file. Either
 +from the database or from the working directory. Thus there are four
 +main combinations: 
 +
 + 1) working directory -> index
 +
 +	You update the index with information from the working directory
 +	with the "update-cache" command.  You generally update the index
 +	information by just specifying the filename you want to update,
 +	like so:
 +
 +		update-cache filename
 +
 +	but to avoid common mistakes with filename globbing etc, the
 +	command will not normally add totally new entries or remove old
 +	entries, ie it will normally just update existing cache entryes.
 +
 +	To tell git that yes, you really do realize that certain files
 +	no longer exist in the archive, or that new files should be
 +	added, you should use the "--remove" and "--add" flags
 +	respectively.
 +
 +	NOTE! A "--remove" flag does _not_ mean that subsequent
 +	filenames will necessarily be removed: if the files still exist
 +	in your directory structure, the index will be updated with
 +	their new status, not removed. The only thing "--remove" means
 +	is that update-cache will be considering a removed file to be a
 +	valid thing, and if the file really does not exist any more, it
 +	will update the index accordingly. 
 +
 +	As a special case, you can also do "update-cache --refresh",
 +	which will refresh the "stat" information of each index to match
 +	the current stat information. It will _not_ update the object
 +	status itself, and it wil only update the fields that are used
 +	to quickly test whether an object still matches its old backing
 +	store object.
 +
 + 2) index -> object database
 +
 +	You write your current index file to a "tree" object with the
 +	program
 +
 +		write-tree
 +
 +	that doesn't come with any options - it will just write out the
 +	current index into the set of tree objects that describe that
 +	state, and it will return the name of the resulting top-level
 +	tree. You can use that tree to re-generate the index at any time
 +	by going in the other direction:
 +
 + 3) object database -> index
 +
 +	You read a "tree" file from the object database, and use that to
 +	populate (and overwrite - don't do this if your index contains
 +	any unsaved state that you might want to restore later!) your
 +	current index.  Normal operation is just
 +
 +		read-tree <sha1 of tree>
 +
 +	and your index file will now be equivalent to the tree that you
 +	saved earlier. However, that is only your _index_ file: your
 +	working directory contents have not been modified.
 +
 + 4) index -> working directory
 +
 +	You update your working directory from the index by "checking
 +	out" files. This is not a very common operation, since normally
 +	you'd just keep your files updated, and rather than write to
 +	your working directory, you'd tell the index files about the
 +	changes in your working directory (ie "update-cache").
 +
 +	However, if you decide to jump to a new version, or check out
 +	somebody elses version, or just restore a previous tree, you'd
 +	populate your index file with read-tree, and then you need to
 +	check out the result with
 +
 +		checkout-cache filename
 +
 +	or, if you want to check out all of the index, use "-a".
 +
 +	NOTE! checkout-cache normally refuses to overwrite old files, so
 +	if you have an old version of the tree already checked out, you
 +	will need to use the "-f" flag (_before_ the "-a" flag or the
 +	filename) to _force_ the checkout.
 +
 +
 +Finally, there are a few odds and ends which are not purely moving from
 +one representation to the other:
 +
 + 5) Tying it all together
 +
 +	To commit a tree you have instantiated with "write-tree", you'd
 +	create a "commit" object that refers to that tree and the
 +	history behind it - most notably the "parent" commits that
 +	preceded it in history. 
 +
 +	Normally a "commit" has one parent: the previous state of the
 +	tree before a certain change was made. However, sometimes it can
 +	have two or more parent commits, in which case we call it a
 +	"merge", due to the fact that such a commit brings together
 +	("merges") two or more previous states represented by other
 +	commits. 
 +
 +	In other words, while a "tree" represents a particular directory
 +	state of a working directory, a "commit" represents that state
 +	in "time", and explains how we got there. 
 +
 +	You create a commit object by giving it the tree that describes
 +	the state at the time of the commit, and a list of parents:
 +
 +		commit-tree <tree> -p <parent> [-p <parent2> ..]
 +
 +	and then giving the reason for the commit on stdin (either
 +	through redirection from a pipe or file, or by just typing it at
 +	the tty). 
 +
 +	commit-tree will return the name of the object that represents
 +	that commit, and you should save it away for later use.
 +	Normally, you'd commit a new "HEAD" state, and while git doesn't
 +	care where you save the note about that state, in practice we
 +	tend to just write the result to the file ".git/HEAD", so that
 +	we can always see what the last committed state was.
 +
 + 6) Examining the data
 +
 +	You can examine the data represented in the object database and
 +	the index with various helper tools. For every object, you can
 +	use "cat-file" to examine details about the object:
 +
 +		cat-file -t <objectname>
 +
 +	shows the type of the object, and once you have the type (which
 +	is usually implicit in where you find the object), you can use
 +
 +		cat-file blob|tree|commit <objectname>
 +
 +	to show its contents. NOTE! Trees have binary content, and as a
 +	result there is a special helper for showing that content,
 +	called "ls-tree", which turns the binary content into a more
 +	easily readable form.
 +
 +	It's especially instructive to look at "commit" objects, since
 +	those tend to be small and fairly self-explanatory. In
 +	particular, if you follow the convention of having the top
 +	commit name in ".git/HEAD", you can do
 +
 +		cat-file commit $(cat .git/HEAD)
 +
 +	to see what the top commit was.
 +
 + 7) Merging multiple trees
 +
 +	Git helps you do a three-way merge, which you can expand to
 +	n-way by repeating the merge procedure arbitrary times until you
 +	finally "commit" the state.  The normal situation is that you'd
 +	only do one three-way merge (two parents), and commit it, but if
 +	you like to, you can do multiple parents in one go.
 +
 +	To do a three-way merge, you need the two sets of "commit"
 +	objects that you want to merge, use those to find the closest
 +	common parent (a third "commit" object), and then use those
 +	commit objects to find the state of the directory ("tree"
 +	object) at these points. 
 +
 +	To get the "base" for the merge, you first look up the common
 +	parent of two commits with
 +
 +		merge-base <commit1> <commit2>
 +
 +	which will return you the commit they are both based on.  You
 +	should now look up the "tree" objects of those commits, which
 +	you can easily do with (for example)
 +
 +		cat-file commit <commitname> | head -1
 +
 +	since the tree object information is always the first line in a
 +	commit object. 
 +
 +	Once you know the three trees you are going to merge (the one
 +	"original" tree, aka the common case, and the two "result" trees,
 +	aka the branches you want to merge), you do a "merge" read into
 +	the index. This will throw away your old index contents, so you
 +	should make sure that you've committed those - in fact you would
 +	normally always do a merge against your last commit (which
 +	should thus match what you have in your current index anyway).
 +	To do the merge, do
 +
 +		read-tree -m <origtree> <target1tree> <target2tree>
 +
 +	which will do all trivial merge operations for you directly in
 +	the index file, and you can just write the result out with
 +	"write-tree". 
 +
 +	NOTE! Because the merge is done in the index file, and not in
 +	your working directory, your working directory will no longer
 +	match your index. You can use "checkout-cache -f -a" to make the
 +	effect of the merge be seen in your working directory.
 +
 +	NOTE2! Sadly, many merges aren't trivial. If there are files
 +	that have been added.moved or removed, or if both branches have
 +	modified the same file, you will be left with an index tree that
 +	contains "merge entries" in it. Such an index tree can _NOT_ be
 +	written out to a tree object, and you will have to resolve any
 +	such merge clashes using other tools before you can write out
 +	the result.
 +
 +	[ fixme: talk about resolving merges here ]
 +

 commit 4bb04f2190d526f8917663f0be62d8026e1ed100
 Author: Linus Torvalds <[email protected]>
 Date:   Mon Apr 11 15:47:57 2005 -0700

    Rename ".dircache" directory to ".git"
    
    I started out calling the tool "dircache". That's clearly moronic.

 diff --git a/README b/README
 --- a/README
 +++ b/README
 @@ -15,154 +15,154 @@
 This is a stupid (but extremely fast) directory content manager.  It
 doesn't do a whole lot, but what it _does_ do is track directory
 contents efficiently. 
 
 There are two object abstractions: the "object database", and the
 "current directory cache".
 
 	The Object Database (SHA1_FILE_DIRECTORY)
 
 The object database is literally just a content-addressable collection
 of objects.  All objects are named by their content, which is
 approximated by the SHA1 hash of the object itself.  Objects may refer
 to other objects (by referencing their SHA1 hash), and so you can build
 up a hierarchy of objects. 
 
 There are several kinds of objects in the content-addressable collection
 database.  They are all in deflated with zlib, and start off with a tag
 of their type, and size information about the data.  The SHA1 hash is
 always the hash of the _compressed_ object, not the original one.
 
 In particular, the consistency of an object can always be tested
 independently of the contents or the type of the object: all objects can
 be validated by verifying that (a) their hashes match the content of the
 file and (b) the object successfully inflates to a stream of bytes that
 forms a sequence of <ascii tag without space> + <space> + <ascii decimal
 size> + <byte\0> + <binary object data>. 
 
 BLOB: A "blob" object is nothing but a binary blob of data, and doesn't
 refer to anything else.  There is no signature or any other verification
 of the data, so while the object is consistent (it _is_ indexed by its
 sha1 hash, so the data itself is certainly correct), it has absolutely
 no other attributes.  No name associations, no permissions.  It is
 purely a blob of data (ie normally "file contents"). 
 
 TREE: The next hierarchical object type is the "tree" object.  A tree
 object is a list of permission/name/blob data, sorted by name.  In other
 words the tree object is uniquely determined by the set contents, and so
 two separate but identical trees will always share the exact same
 object. 
 
 Again, a "tree" object is just a pure data abstraction: it has no
 history, no signatures, no verification of validity, except that the
 contents are again protected by the hash itself.  So you can trust the
 contents of a tree, the same way you can trust the contents of a blob,
 but you don't know where those contents _came_ from. 
 
 Side note on trees: since a "tree" object is a sorted list of
 "filename+content", you can create a diff between two trees without
 actually having to unpack two trees.  Just ignore all common parts, and
 your diff will look right.  In other words, you can effectively (and
 efficiently) tell the difference between any two random trees by O(n)
 where "n" is the size of the difference, rather than the size of the
 tree. 
 
 Side note 2 on trees: since the name of a "blob" depends entirely and
 exclusively on its contents (ie there are no names or permissions
 involved), you can see trivial renames or permission changes by noticing
 that the blob stayed the same.  However, renames with data changes need
 a smarter "diff" implementation. 
 
 CHANGESET: The "changeset" object is an object that introduces the
 notion of history into the picture.  In contrast to the other objects,
 it doesn't just describe the physical state of a tree, it describes how
 we got there, and why. 
 
 A "changeset" is defined by the tree-object that it results in, the
 parent changesets (zero, one or more) that led up to that point, and a
 comment on what happened. Again, a changeset is not trusted per se:
 the contents are well-defined and "safe" due to the cryptographically
 strong signatures at all levels, but there is no reason to believe that
 the tree is "good" or that the merge information makes sense. The
 parents do not have to actually have any relationship with the result,
 for example.
 
 Note on changesets: unlike real SCM's, changesets do not contain rename
 information or file mode chane information.  All of that is implicit in
 the trees involved (the result tree, and the result trees of the
 parents), and describing that makes no sense in this idiotic file
 manager.
 
 TRUST: The notion of "trust" is really outside the scope of "git", but
 it's worth noting a few things. First off, since everything is hashed
 with SHA1, you _can_ trust that an object is intact and has not been
 messed with by external sources. So the name of an object uniquely
 identifies a known state - just not a state that you may want to trust.
 
 Furthermore, since the SHA1 signature of a changeset refers to the
 SHA1 signatures of the tree it is associated with and the signatures
 of the parent, a single named changeset specifies uniquely a whole
 set of history, with full contents. You can't later fake any step of
 the way once you have the name of a changeset.
 
 So to introduce some real trust in the system, the only thing you need
 to do is to digitally sign just _one_ special note, which includes the
 name of a top-level changeset.  Your digital signature shows others that
 you trust that changeset, and the immutability of the history of
 changesets tells others that they can trust the whole history.
 
 In other words, you can easily validate a whole archive by just sending
 out a single email that tells the people the name (SHA1 hash) of the top
 changeset, and digitally sign that email using something like GPG/PGP.
 
 In particular, you can also have a separate archive of "trust points" or
 tags, which document your (and other peoples) trust.  You may, of
 course, archive these "certificates of trust" using "git" itself, but
 it's not something "git" does for you. 
 
 Another way of saying the same thing: "git" itself only handles content
 integrity, the trust has to come from outside. 
 
 -	Current Directory Cache (".dircache/index")
 +	Current Directory Cache (".git/index")
 
 The "current directory cache" is a simple binary file, which contains an
 efficient representation of a virtual directory content at some random
 time.  It does so by a simple array that associates a set of names,
 dates, permissions and content (aka "blob") objects together.  The cache
 is always kept ordered by name, and names are unique at any point in
 time, but the cache has no long-term meaning, and can be partially
 updated at any time. 
 
 In particular, the "current directory cache" certainly does not need to
 be consistent with the current directory contents, but it has two very
 important attributes:
 
  (a) it can re-generate the full state it caches (not just the directory
      structure: through the "blob" object it can regenerate the data too)
 
      As a special case, there is a clear and unambiguous one-way mapping
      from a current directory cache to a "tree object", which can be
      efficiently created from just the current directory cache without
      actually looking at any other data.  So a directory cache at any
      one time uniquely specifies one and only one "tree" object (but
      has additional data to make it easy to match up that tree object
      with what has happened in the directory)
     
 
 and
 
  (b) it has efficient methods for finding inconsistencies between that
      cached state ("tree object waiting to be instantiated") and the
      current state. 
 
 Those are the two ONLY things that the directory cache does.  It's a
 cache, and the normal operation is to re-generate it completely from a
 known tree object, or update/compare it with a live tree that is being
 developed.  If you blow the directory cache away entirely, you haven't
 lost any information as long as you have the name of the tree that it
 described. 
 
 (But directory caches can also have real information in them: in
 particular, they can have the representation of an intermediate tree
 that has not yet been instantiated.  So they do have meaning and usage
 outside of caching - in one sense you can think of the current directory
 cache as being the "work in progress" towards a tree commit).

 commit e83c5163316f89bfbde7d9ab23ca2e25604af290
 Author: Linus Torvalds <[email protected]>
 Date:   Thu Apr 7 15:13:13 2005 -0700

    Initial revision of "git", the information manager from hell

 diff --git a/README b/README
 --- /dev/null
 +++ b/README
 @@ -0,0 +15,154 @@
 +This is a stupid (but extremely fast) directory content manager.  It
 +doesn't do a whole lot, but what it _does_ do is track directory
 +contents efficiently. 
 +
 +There are two object abstractions: the "object database", and the
 +"current directory cache".
 +
 +	The Object Database (SHA1_FILE_DIRECTORY)
 +
 +The object database is literally just a content-addressable collection
 +of objects.  All objects are named by their content, which is
 +approximated by the SHA1 hash of the object itself.  Objects may refer
 +to other objects (by referencing their SHA1 hash), and so you can build
 +up a hierarchy of objects. 
 +
 +There are several kinds of objects in the content-addressable collection
 +database.  They are all in deflated with zlib, and start off with a tag
 +of their type, and size information about the data.  The SHA1 hash is
 +always the hash of the _compressed_ object, not the original one.
 +
 +In particular, the consistency of an object can always be tested
 +independently of the contents or the type of the object: all objects can
 +be validated by verifying that (a) their hashes match the content of the
 +file and (b) the object successfully inflates to a stream of bytes that
 +forms a sequence of <ascii tag without space> + <space> + <ascii decimal
 +size> + <byte\0> + <binary object data>. 
 +
 +BLOB: A "blob" object is nothing but a binary blob of data, and doesn't
 +refer to anything else.  There is no signature or any other verification
 +of the data, so while the object is consistent (it _is_ indexed by its
 +sha1 hash, so the data itself is certainly correct), it has absolutely
 +no other attributes.  No name associations, no permissions.  It is
 +purely a blob of data (ie normally "file contents"). 
 +
 +TREE: The next hierarchical object type is the "tree" object.  A tree
 +object is a list of permission/name/blob data, sorted by name.  In other
 +words the tree object is uniquely determined by the set contents, and so
 +two separate but identical trees will always share the exact same
 +object. 
 +
 +Again, a "tree" object is just a pure data abstraction: it has no
 +history, no signatures, no verification of validity, except that the
 +contents are again protected by the hash itself.  So you can trust the
 +contents of a tree, the same way you can trust the contents of a blob,
 +but you don't know where those contents _came_ from. 
 +
 +Side note on trees: since a "tree" object is a sorted list of
 +"filename+content", you can create a diff between two trees without
 +actually having to unpack two trees.  Just ignore all common parts, and
 +your diff will look right.  In other words, you can effectively (and
 +efficiently) tell the difference between any two random trees by O(n)
 +where "n" is the size of the difference, rather than the size of the
 +tree. 
 +
 +Side note 2 on trees: since the name of a "blob" depends entirely and
 +exclusively on its contents (ie there are no names or permissions
 +involved), you can see trivial renames or permission changes by noticing
 +that the blob stayed the same.  However, renames with data changes need
 +a smarter "diff" implementation. 
 +
 +CHANGESET: The "changeset" object is an object that introduces the
 +notion of history into the picture.  In contrast to the other objects,
 +it doesn't just describe the physical state of a tree, it describes how
 +we got there, and why. 
 +
 +A "changeset" is defined by the tree-object that it results in, the
 +parent changesets (zero, one or more) that led up to that point, and a
 +comment on what happened. Again, a changeset is not trusted per se:
 +the contents are well-defined and "safe" due to the cryptographically
 +strong signatures at all levels, but there is no reason to believe that
 +the tree is "good" or that the merge information makes sense. The
 +parents do not have to actually have any relationship with the result,
 +for example.
 +
 +Note on changesets: unlike real SCM's, changesets do not contain rename
 +information or file mode chane information.  All of that is implicit in
 +the trees involved (the result tree, and the result trees of the
 +parents), and describing that makes no sense in this idiotic file
 +manager.
 +
 +TRUST: The notion of "trust" is really outside the scope of "git", but
 +it's worth noting a few things. First off, since everything is hashed
 +with SHA1, you _can_ trust that an object is intact and has not been
 +messed with by external sources. So the name of an object uniquely
 +identifies a known state - just not a state that you may want to trust.
 +
 +Furthermore, since the SHA1 signature of a changeset refers to the
 +SHA1 signatures of the tree it is associated with and the signatures
 +of the parent, a single named changeset specifies uniquely a whole
 +set of history, with full contents. You can't later fake any step of
 +the way once you have the name of a changeset.
 +
 +So to introduce some real trust in the system, the only thing you need
 +to do is to digitally sign just _one_ special note, which includes the
 +name of a top-level changeset.  Your digital signature shows others that
 +you trust that changeset, and the immutability of the history of
 +changesets tells others that they can trust the whole history.
 +
 +In other words, you can easily validate a whole archive by just sending
 +out a single email that tells the people the name (SHA1 hash) of the top
 +changeset, and digitally sign that email using something like GPG/PGP.
 +
 +In particular, you can also have a separate archive of "trust points" or
 +tags, which document your (and other peoples) trust.  You may, of
 +course, archive these "certificates of trust" using "git" itself, but
 +it's not something "git" does for you. 
 +
 +Another way of saying the same thing: "git" itself only handles content
 +integrity, the trust has to come from outside. 
 +
 +	Current Directory Cache (".dircache/index")
 +
 +The "current directory cache" is a simple binary file, which contains an
 +efficient representation of a virtual directory content at some random
 +time.  It does so by a simple array that associates a set of names,
 +dates, permissions and content (aka "blob") objects together.  The cache
 +is always kept ordered by name, and names are unique at any point in
 +time, but the cache has no long-term meaning, and can be partially
 +updated at any time. 
 +
 +In particular, the "current directory cache" certainly does not need to
 +be consistent with the current directory contents, but it has two very
 +important attributes:
 +
 + (a) it can re-generate the full state it caches (not just the directory
 +     structure: through the "blob" object it can regenerate the data too)
 +
 +     As a special case, there is a clear and unambiguous one-way mapping
 +     from a current directory cache to a "tree object", which can be
 +     efficiently created from just the current directory cache without
 +     actually looking at any other data.  So a directory cache at any
 +     one time uniquely specifies one and only one "tree" object (but
 +     has additional data to make it easy to match up that tree object
 +     with what has happened in the directory)
 +    
 +
 +and
 +
 + (b) it has efficient methods for finding inconsistencies between that
 +     cached state ("tree object waiting to be instantiated") and the
 +     current state. 
 +
 +Those are the two ONLY things that the directory cache does.  It's a
 +cache, and the normal operation is to re-generate it completely from a
 +known tree object, or update/compare it with a live tree that is being
 +developed.  If you blow the directory cache away entirely, you haven't
 +lost any information as long as you have the name of the tree that it
 +described. 
 +
 +(But directory caches can also have real information in them: in
 +particular, they can have the representation of an intermediate tree
 +that has not yet been instantiated.  So they do have meaning and usage
 +outside of caching - in one sense you can think of the current directory
 +cache as being the "work in progress" towards a tree commit).