Prompt: Ensure "git status" starts with "# "

Git 1.8.5 removed the comment characters from "git status" output and
broke my shell prompt. Since I'm feeling luc^H^Hazy, I'll just make sure
that whatever Git version I have, the comment characters are there.

Yes, I know about --porcelain, but it doesn't output all information I
need, and I don't want to introduce additional commands at the moment.