rbt should find repositories based on mirror_path

"rbt post -u" doesn't work if the repository is being identified using a
mirror_path - get_repository_id returns None which causes an error trying to do
the update.

The patch matches the logic in commands/setup_repo.py and clients/svn.py.

Testing Done:
Tested locally against reviewboard 1.7 - repo with the path of
http://svn.DOMAIN/svn/foo and mirror_path of http://svn/svn/foo works with `rbt
post -u` when my local checkout is http://svn/svn/foo

Reviewed at https://reviews.reviewboard.org/r/5468/