R1:ec89c7d63e26

Add an "Unreachable" flag for commits and revive them during discovery

Summary:
Ref T9028. This is the easy part of dealing with deleted commits:

- Add a flag for unreachable commits (nothing sets this flag yet).
- Ignore unreachable commits when querying for known commits during discovery, so we pretend they do not exist.
- When recording a commit, try just reviving an existing unreachable commit first. If that works, bail out.

Test Plan:
- Artificially marked a commit as unreachable with raw…