Edit a merge commit with git rebase

Updated answer for 2020:

You can force git rebase -i to stop at the merge commit via the break command (added in Git 2.20). You can then edit the merge commit as desired via git commit --amend.

Detailed steps:

1. Run git rebase -i --rebase-merges $ancestor_of_merge_commit
2. Find the merge commit you want to edit in the todo list.
3. Insert a new line after the merge commit that contains only break (or b).
4. Save your changes and exit your editor. Git rebase will check out the merge commit, print something like the following, then return you to your prompt:

   Stopped at fb91fab (Merge branch 'foo' into bar)
   

5. Use git commit --amend to edit the merge commit as desired.
6. Run git rebase --continue when you are done editing the merge commit.

Alternative approach if you do not have any merge commits after the commit you want to edit:

1. Run git rebase -i $id_of_merge_commit
2. Insert a new line at the top of the todo list that contains only break (or b).
3. Save your changes and exit your editor. Git rebase will check out the merge commit, print something like the following, then return you to your prompt:

   Stopped at fb91fab (Merge branch 'foo' into bar)
   

4. Use git commit --amend to edit the merge commit as desired.
5. Run git rebase --continue when you are done editing the merge commit.

Original answer from 2012 (pre-break):

Git does not make it easy to do interactive rebases when merges are involved. The -p option uses the -i mechanism internally, so mixing the two doesn't really work.

However, git rebase is just an automated way to do lots of cherry-picks. You can replicate its behavior by manually cherry-picking to get a bit more control over the process. It's less convenient and more prone to human error, but possible.

This is the approach I suggest:

1. use git rebase to get to the commit after the merge (the child of the merge)
2. use git reset --hard HEAD^ to manually get to the merge
3. use git commit --amend to repair the merge
4. use git cherry-pick to get back to the commit after the merge
5. use git rebase --continue to finish

Here are the specific steps:

1. Note the SHA1 ID of the merge commit you want to modify. For discussion, suppose it is deadbeef.
2. Note the SHA1 ID of the commit right after the merge commit you want to modify (the merge commit's child). Suppose it is facef00d.
3. Run git rebase -i deadbeef.
4. Select facef00d for editing.
5. When rebase returns you to a prompt to edit facef00d, run git reset --hard HEAD^. You should now be at deadbeef (git rev-parse HEAD should print deadbeef).
6. Make your edits to fix the incorrect merge conflict and use git add to stage them.
7. Run git commit --amend to fuse the staged fix with the bad merge commit. The result will now have a different SHA1 (not deadbeef).
8. Run git cherry-pick facef00d to apply the changes made by facef00d to the fixed merge commit.
9. Run git rebase --continue to finish.

May be easier to create a fixup commit 'D' then use 'git rebase -p -i <blah>' to reorder 'D' right after 'B' and squash it into 'B'.

pick A
pick B  <- merge commit to ammend
fixup D
pick C

This is much easier now with the --rebase-merges option available in Git 2.22 and above. This option preserves the merge topology, and works with interactive rebases.

It'll look something like this, assuming B is the merge commit to amend:

label onto

... some branch definitions ...

reset onto
merge -C B branch-name # Merge branch 'B' into whatever
pick C

You can now insert a b (or break) in between the merge and the pick:

merge -C B branch-name # Merge branch 'xyz' into whatever
break
pick C

At the break commit --amend the merge, and then continue the rebase.

This also works with a fixup commit. For example, lets say the commit with the fix for the merge is D. You can move your fixup commit D to immediately after the merge commit:

merge -C B branch-name # Merge branch 'xyz' into whatever
fixup D
pick C

See the Rebasing Merges section of the git-merge man page.