R1:a15130b47c6d

Add a maintenance script for reconciling repositories to disk state

Summary:
@rguerin ran into an issue in his install where Phabricator appears to have
discovered commits which no longer exist, and thus is failing to proceed with
its repository import.

It's not clear how we got into this state. Previously, it was possible by, e.g.,
parsing a different repository's working copy and then switching them back, but
there are now safeguards against that.

I'm taking a three-pronged approach to try to sort…