Optionally allow popping up an editor

[Justin]

Add flag so that the commit message can be further updated in an editor. I find it convenient to specify the cmt msg in the command line, but for git it would be nice to be able to either add additional lines or reformat the message a little.

[Michał Górny]

I think this actually belongs to repoman, i.e. we call repoman -a and it brings up editor instead of dumb yes/no query. The way it does without commit message. Not that I see a reason to use repo-commit these days anymore.