Trying to excise some commits

git-excise-attempt.md

I have a project with a long git history. I want to open-source it, but there's a long section at the beginning that I want to remove first.

*    ddfe660 - (HEAD, origin/master)
|
|    (many hundreds of commits, including merges)
|
*    420b801 - (the intended beginning of the new history)
*    00a3516 - (the most recent commit I want squashed out)
|
|    (many hundreds of commits, including merges)
|
*    6b6fc1e - first commit

I pseudo-git, I want git squash 6b6fc1e..420b801. That is, I want a new first commit that is the sum of that whole range, with the commit message from 420b801.

Here are some things I've tried:

1. Interactive rebase: git rebase -i 6b6fc1e, using fixup for all of 6b6fc1e..00a3516 and squash for 420b801. This failed as soon as I got to a merge commit in the first range because I don't know how to resolve those conflicts.
2. Interactive rebase with --preserve-merge: git rebase -p -i 6b6fc1e. This had the same problem.
3. git rebase -p --root 420b801. This had the same problem.
4. Fiddling with --root: git checkout 6b6fc1e && git reset --soft 420b801 && git commit --amend, as suggested here. That didn't seem to remove any of the commits in the range I want to squash.

I'm certain there's a more elegant way to do this. However, one way that would probably work is as follows. (This is off the top of my head so I might have mistakes.)

1. Make a branch from the last commit in the range you want to squash.
2. In this branch git reset FIRST_COMMIT. This should leave you with all the changes made between those two commits but unstaged.
3. Make a new commit with all of these changes. If this went right, what was originally a ton of commits should just be one or two.
4. Rebase master (or better yet make another branch from master) on this branch.

@wagenet part one worked great:

git checkout -b excise # start a new branch
git reset --hard 420b801 # point it to the new first commit
git reset 6b6fc1e # stage the whole history
git commit -m "new first commit"
git tag tmp

That left me with a new tree with exactly one commit containing exactly what I want!

Then I tried part two:

git checkout -b new-master
git reset --hard origin/master
git rebase --onto tmp

I can't exactly explain what that did, but it seemed to have made master point to just the one-commit tree tmp.

Another attempt:

git checkout -b new-master
git reset --hard origin/master
git rebase tmp

This had the merge-conflict problems.

But a third seemed to go better:

git checkout -b new-master
git reset --hard origin/master
git rebase -p tmp

I need to do some verification, but this seems to have worked!

Nope, I'm wrong. The third was the same as the first. I lost 420b801.. ddfe660. Maybe I can cherry-pick them now?

Next attempt:

git checkout -b new-master
git reset --hard origin/master
git rebase -p -i 6b6fc1e

and then remove all of the lines of 420b801.. tmp and replace them with the single line pick tmp. I still get merge conflicts, but many fewer and more recent, so there's a hope I could resolve them.

Don't do a hard reset. The hard reset will remove all the changes, which you don't want to do. The git reset without hard will change the HEAD and unstage all the changes.

Sorry, I don't understand. I want to retain all the recent history. I don't just want to end up with a one-commit tree.

Now trying git rebase -i -s recursive -X theirs 6b6fc1e and replacing the first section with pick tmp...

That didn't work because git told me it didn't understand the theirs strategy.

Just tried

git checkout tmp
git cherry-pick 420b801.. ddfe660

Which kept telling me it couldn't cherry-pick merge commits without an -m. Passing that to the range didn't work. I needed to do it at each merge. That's a lot of steps, so I wrote a Ruby script:

require 'open3'

REGEX = /Commit ([0-9a-f]+) is a merge but no -m option was given\./

def step
  puts 'git cherry-pick --continue'
  out, err, status = Open3.capture3('git cherry-pick --continue')
  md = REGEX.match(err)

  return false if md.nil?

  puts "git cherry-pick #{md[1]} -m 1"
  `git cherry-pick #{md[1]} -m 1`

  puts "git commit --allow-empty -C #{md[1]}"
  `git commit --allow-empty -C #{md[1]}`

  true
end

needs_another = true

while needs_another
  needs_another = step
end

But that just got me to a merge conflict it couldn't resolve.

What ended up working:

$ git checkout master
$ git checkout -b new-master
$ git rebase -p -i --root

Then delete all the lines I wanted to remove and add pick tmp at the top. When there were rebase conflicts, they would look like

CONFLICT (rename/delete): app/pods/components/nav-item/component.js deleted in HEAD and renamed in a581711d5119e24f2e26e65795a7c2b776efaa46. Version a581711d5119e24f2e26e65795a7c2b776efaa46 of app/pods/components/nav-item/component.js left in tree.
Automatic merge failed; fix conflicts and then commit the result.
Error redoing merge 150b353693d1f31906f085acad17546128ae41a0
$ git add app/pods/components/nav-item/component.js
$ git rebase --continue

For each of these, I just did

$ git checkout 150b353693d1f31906f085acad17546128ae41a0 -- .
$ git add .
$ git rebase --continue