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 SQL.
• Verified it said "deleted: unreachable" in the UI.
• Ran repository discover --trace --verbose.
• Saw the discovery process ignore the commit when filling the cache.
• Saw the discovery process revive the commit instead of trying to record it again.
• Web UI now shows the commit as normal.
• Running repository discover again doesn't make any further changes.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9028

Differential Revision: https://secure.phabricator.com/D16130