Add "bin/repository mark-reachable" for fixing commit reachability flags

Summary:
Ref T9028. This corrects the reachability of existing commits in a repository.

In particular, it can be used to mark deleted commits as unreachable.

Test Plan:

• Ran it on a bad repository, with bad args, etc.
• Ran it on a clean repo, got no changes.
• Marked a reachable commit as unreachable, ran script, got it marked reachable.
• Started deleting tags and branches from the local working copy while running the script, saw greater parts of the repository get marked unreachable.
• Pulled repository again, everything automatically revived.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9028

Differential Revision: https://secure.phabricator.com/D16132