Re: How to split a big commit

[Ramkumar Ramachandra <artagnon@gmail.com>]

Hi Nguyen,

Nguyen Thai Ngoc Duy writes:
> After a lot of small commits and a few mixed up large commits, it was
> too messy that I merged them all into one big commit then started
> spliting it into smaller, reasonable patches. Just wonder if anybody
> else faces the same thing and how they deal with it. I used "git reset
> --soft <big commit>^" and "git add -N" because there were new files,
> but it was clumsy.

To split a big commit, I simply reverse-diff-apply the changes that I
don't want in the big commit, and I stage all changes.  Then, I again
reverse-diff-apply all the staged changes, and keep that in the
unstaged portion.  Your repository should look like this now:

commit bigcommit
diff --git a/bigfile b/bigfile
+Desirable change
+Undesirable change

Staged changes:
diff --git a/bigfile b/bigfile
-Undesirable change

Unstaged changes:
diff --git a/bigfile b/bigfile
+Undesirable change

Now simply 'commit --amend' to remove the undesirable change from the
bigcommit, and commit the unstaged changes to produce a new
commit. Final result:

commit bigcommit
diff --git a/bigfile b/bigfile
+Desirable change

commit newcommit
diff --git a/bigfile b/bigfile
+Undesirable change

I can afford to do this kind of jugglery very easily because I use
Magit with Emacs.  After highlighting the relevant portions,
reverse-diff-apply is one keystroke :) But yes, you're probably right-
there should be an easier way to do this.  Maybe a 'git split' that
allows you to interactively select the portions of an existing commit
that you'd like to exclude from the commit and turn into a new commit?

Hope that helps.

-- Ram