Amending the message of Git commit made before a merge

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.