More new git tricks:
A 'git rebase -i' allows you to edit a past commit. However, it will 'serialize' the history, losing any branch/merge 'structure' there might have been.
A 'git rebase -i --rebase-merges' keeps the branch/merge 'structure', however, it is not obvious how to 'edit' a merge commit.
You can, though: you can insert a 'break' in the rebase to edit and 'commit --amend' a merge commit.
https://stackoverflow.com/questions/9930637/edit-a-merge-commit-with-git-rebase