Warn users about a dirty working directory.

If a user posts a change with no revision arguments, they probably want their
"current" state to be the change. We've had a little bit of confusion with Git
users who hadn't committed their changes to a branch and were upset that their
working tree wasn't posted (like it would be with svn, for example).

This change adds a warning when the user invokes rbt post without explicit
revision arguments and has a dirty working directory.

Testing done:
Posted this change with some other stuff in my working directory and saw the
warning.

Fixes bug 3120

Reviewed at https://reviews.reviewboard.org/r/5132/