[PATCH RFC] commit: add --reuse-message for keeping the old commit message during amend

[Durham Goode]

>  What about something generic like --no-editor? It's not as explicit
>  but we already have --editor which forces an editor, so this will do
>  the opposite.
>  
>  It also opens up the possibility of appearing in other commands with
>  similar semantics but without being directly related to amend.

I think the name must imply some relationship with another commit, so it's
semi intuitive that it's related to --amend. I think a user would be more
likely to try --no-editor without --amend, and be confused when it failed.