This was a new one on me… and I needed it recently. I accidentally committed a change with a totally nonsensical message since I accidentally copy-and-pasted some code into the message field of my commit, which automatically commenced.
Thankfully, there is a way to update it before you push (that seems to be the key to success here… don’t push it after you do the commit with the bogus message):
git commit --amend -m "New commit message"
I’m just relieved that this can be done… and a little surprised — given how grumpy and curmudgeonly Git feels sometimes.