Add a --repair flag to "bin/repository discover"

Summary:
If a repository is missing commits because they mysteriously vanished, there's no reasonable way to get them back right now. Provide a way to ignore the state in the database and rediscover the entire repository unconditionally.

We don't queue any reparses or anything, but when I move reparse into this script we can hook things up or something. This generally shouldn't be too important anyway.

Test Plan: Ran repository discover --repair on Phabricator.

Reviewers: jungejason, nh, vrana

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D2850