In Magit place the cursor on the commit in question press r w. The current message appears in a buffer. Edit it and press C-c C-c when done. Yes, it uses the horrible git rebase, but as you said, nothing can go wrong when editing a commit message, so that is mostly an implementation details. (Admittedly it only works if the commit is reachable from HEAD.)
In Magit place the cursor on the commit in question press
r w
. The current message appears in a buffer. Edit it and pressC-c C-c
when done. Yes, it uses the horriblegit rebase
, but as you said, nothing can go wrong when editing a commit message, so that is mostly an implementation details. (Admittedly it only works if the commit is reachable from HEAD.)