Feature Request: `git commit --amend-to`

`git commit --amend` is very useful for adding changes to the previous commit quickly.

Unfortunately, it is more difficult to add changes to older commits. To do so I have to enter interactive rebase, mark the commit to change, make the changes, amend the previous commit, and continue the interactive rebase.

It would be nice if there was a single command to do this like `git commit --amend-to SHA`

I have solved this issue for myself with this handy alias I found online ``` git config --global alias.amend-to '!f() { SHA=`git rev-parse "$1"`; git commit --fixup "$SHA" && GIT_SEQUENCE_EDITOR=true git rebase --interactive --autosquash "$SHA^"; }; f' ```

I think it would be beneficial if a command `git commit --amend-to` could be officially added.

Thanks, Aidan Gallagher

On Fri, Sep 24, 2021 at 9:08 AM Aidan Gallagher <aidgal2@gmail.com> wrote:

since v2.31.0 you can do instead :

  git commit --fixup reword:$SHA && EDITOR=true git rebase
--interactive --autosquash "$SHA^"

granted it is not 1 command, but usually I find it useful to do several of those and then one single rebase at the end.

Carlo

Carlo Arenas <carenas@gmail.com> writes:

This is fairly similar to what I use, though I use this through the Magit Emacs plugin.

My concern with "--amend-to" is that the semantics aren't as simple as --amend.

With --amend, you know you're working on the branch tip, so it's relatively simple to discard the last commit and create a new one.

With something like --amend-to, you aren't just modifying a single commit, you are also introducing a potential merge conflict with every commit after that. You would have to provide some kind of facility for users to fix the merge conflicts. A command like git rebase --autosquash does a good job at communicating to users that they are actually doing a rebase and they need to be prepared to fix problems through a rebase UX. However, git commit --amend-to communicates none of that. A user who takes a cursory glance at git commit --amend-to has no idea that they are potentially comitting to a rebase.

I personally think the current UI makes sense given how Git works. I also wish that it were easier to do --amend-to, but I think the problem has more to do with how Git manages merges and conflicts and less to do with having shortcuts in the CLI.

On Tue, Sep 28 2021, Glen Choo wrote:

I think that users who'd like an --amend-to would probably be happy with or might want to try out something like "hg absorb", which is something I think we should have stolen in git already, but it's never too late.

I.e. it's a "git commit --squash" on steroids, which tries to find what commit to amend things into.

See [1] (and [2] for an archive of the linked PDF) for a past reference. There's a "git absorb" in Rust that I haven't tried out, but seems interesting[3]. It looks like "absorb" is now part of "hg" itself[4], but I'd looked at & tried it back when it was part of the Facebook-specific patchset to "hg", which I understand has then mostly or entirely gotten upstreamed in some way.

I'd think that for a git implementation we'd want to re-use the engine we've got in range-diff.c, i.e. consider each unstaged hunk and find which hunk/commit in say @{u}.. to squash it into.

1. https://lore.kernel.org/git/877ej0iuhc.fsf@evledraar.gmail.com
2. https://web.archive.org/web/20181003211051/http://files.lihdd.net/hgabsorb-note.pdf
3. https://crates.io/crates/git-absorb
4. https://gregoryszorc.com/blog/2018/11/05/absorbing-commit-changes-in-mercurial-4.8/

Ævar Arnfjörð Bjarmason <avarab@gmail.com> writes:

Agree. I think some "hg absorb" features get at the heart of what I meant, which is that users would benefit from making this merge conflict resolution a lot simpler.

But this is an even bigger step up from what I was suggesting. I blame my own lack of imagination for not even considering this possibility.

To me, what really sells the feature is the "history of lines" approach cited in https://gregoryszorc.com/blog/2018/11/05/absorbing-commit-changes-in-mercurial-4.8/:

  The automatic rewriting logic of hg absorb is implemented by following
  the history of lines. This is fundamentally different from the approach
  taken by hg histedit or git rebase, which tend to rely on merge
  strategies based on the 3-way merge to derive a new version of a file
  given multiple input versions.

Traditional 3-way merge is extremely frustrating when you find yourself touching the same lines over and over in a rebase, and I think "history of lines" maps quite cleanly to how humans think of lines. However I'm not sure if Git has any machinery for doing this.

On 29.09.21 18:55, Glen Choo wrote:

I have never used hg absorb but after reading about its concepts i think it best maps to something like git automatically creating fixup commits for you. I also found https://github.com/tummychow/git-absorb (also untested by me) which claims to do exactly this.

Something like having "git commit -a --fixup" (without any commit hash) create matching fixup commits for all changes i made would already be quite cool.

From what i see from our corporate environments and github/bitbucket workflows though i think most users never even heard about fixup commits or ever did an interactive rebase (this is totally a view from my corporate bubble and might not really represent the average git userbase). Most code review workflows i see don't focus on individual commits and only present the complete final patch (some at least allow you to view individual commits but not much more) and therefore users have no big incentive during review to shape their changes into proper commits. Some tools (like phabricator) even squash the final patch into a single commit by default which i think is a good idea if single commits are of no concern during review anyway.

I wonder if something like an easy to use "absorb" feature (without fixups, rebases) might encourage and produce more clean commit histories even for the mentioned workflows.

On 29/09/2021 17:55, Glen Choo wrote:

Thanks for the link, I had not realized absorb did not use a 3-way merge.

I agree that conflicts get frustrating with fixups but having just played with hg absorb it seems a bit too cavalier to me. The test script at the end of this email sets up the history below. I was surprised it applied the fixup as to my mind it is ambiguous whether 'z' should go before of after 'x' in HEAD~1 and whether 'p' should go before or after 'e' in HEAD~2.

Worktree HEAD HEAD~1 HEAD~2       HEAD HEAD~1 HEAD~2
    a       a    a     +a            a    a     +a
    b       b    b     +b            b    b     +b
   +z      -x   +x     +c      =>   -x   +x     +z
    c       c    c     +d            z    z     +c
    d       d    d     +e            c    d     +d
   +p       f   -e     +f            d   -e     +e
    f            f                   p    p     +p
                                     f    f     +f

I think we could use a modified version of blame to find which commit a line comes from, but bail out in cases where it is ambiguous such as my example above (I think we'd want to require that the two context lines either side of an insertion remain adjacency as we track backwards through the history, and that a group of modified lines remain contiguous). The commits could be rewritten by applying zero context patches taking care to adjust the offsets where the fixup adds or deletes lines (similar to what add -p does)

Best Wishes

Phillip

----- >8 -----
write_lines() {
     printf '%s\n' "$@"
}

repo="$(mktemp -d)" &&
     cd "$repo" &&
     hg init &&
     write_lines >file a b c d e f &&
     hg add file &&
     hg commit -m initial &&
     write_lines >file a b x c d f &&
     hg commit -m one &&
     write_lines >file a b c d f &&
     hg commit -m two &&
     write_lines >file a b z c d p f &&
     hg absorb &&
     hg log -p