Amending the message of Git commit made before a merge

Question

I committed some test code before merging in a remote branch. This merge had a lot of conflicts and took some serious time to put right. So my history looks something like this:

7ab562c Merge from remote branch
... whole load of commits brought across from the remote branch...
f3e71c2 Temporary TESTING COMMIT

The test code is fine, I really just want to change the commit message. Normally I'd go right ahead with a git rebase -i f3e71c2^ (since none of this has been pushed yet), but I've been told by a colleague that this will mess up the merge. I really don't want to mess up the merge :)

Is my colleague correct? And if so, is there anything I can do, or do I just need to live with this history?

Answer 1

You can try a git rebase --preserve-merges --interactive, with:

-p
--preserve-merges

Instead of ignoring merges, try to recreate them.

The BUG section of the man page includes:

The todo list presented by --preserve-merges --interactive does not represent the topology of the revision graph.
Editing commits and rewording their commit messages should work fine, but attempts to reorder commits tend to produce counterintuitive results.

---

As jthill's comment describes (since -p will better preserve merges if conflict resolutions were recorder):

You can retroactively light rerere for a merge:

git config rerere.enabled true
git checkout $merge^1

git merge $merge^2
# for Windows: 
# git merge $merge^^2

git read-tree --reset -u $merge
git commit -m-
git checkout @{-1}

As noted by Soufiane Roui in the comments:

For Windows CMD users, use double caret to point for parents of the merge commit (e.g $merge^^1 instead of $merge^1).
Because the caret is considered as an escape character.

Answer 2

If and only if your colleagues have not pushed/pulled the changes ontop of f3e71c2 elsewhere, this will work. Otherwise I don't know what will happen. Changing the commit message is entirely cosmetic (== metadata change), given that you haven't pushed the commit you want to amend yet, but this could still result in history confusion if your colleagues have pushed/ pulled any part of the history which is atop it.

(thanks to Abizern for pointing out this failure-mode)