"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 rbtools/commands/setup_repo.py and also svn.py

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