Huh. Neither was I. I've been using -C HEAD which has the same affect (reusing the commit from the branch head) but --no-edit makes it more obvious what's going on...sort of. It's still not clear what isn't being edited.
Do you mean the target of the editing? It could also be the commit author or committee perhaps. Maybe the message is the only logical choice, but for long flags, I think more explicit is still better.
