Four commands working on the index

four-commands.md

See commit 6ad6d3d36c5924c8ff502ebbb6a6216df01e7efb in git.git for an update to the README file about these four commands.

+--------------+   update-cache   +--------------+  write-tree   +-----------+
|              | ---------------> |              | ------------> |           |
| working copy |                  | staging area |               | object db |
|              | <--------------- |              | <------------ |           |
+--------------+  checkout-cache  +--------------+   read-tree   +-----------+

(update-cache and checkout-cache were later renamed to *-index.)

In some sense, three of these are exposed through porcelain commands:

plumbing         porcelain
========         =========

update-cache     add, rm

write-tree       commit

checkout-cache   checkout

read-tree        reset

Though that's not 100% accurate, because after doing write-tree, commit also does commit-tree, thus creating the commit object itself, replete with parents and commit comment and whatnot.

Note also that both commit and reset also update HEAD.