R1:02d7bb8604f3

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…