add boolean property: 'files.dirty' if working dir is dirty

Tested with both JGit and native git. Handles both staged and unstaged commits.
I agree that it doesn't match any naming convention, but it's also not part of the commit id or anything like that. This particular property has more to do with the files on the filesystem than anything else though. It's easy enough for me to change the name, but I don't think that "commit.id.files.dirty" (or anything starting "commit.id") is really intuitive. "commit.files.dirty" maybe?