R1:e0b86cc81b39

Add a Mercurial commit discovery daemon

Summary:
Repository import has three major steps:

- Commit discovery (serial)
- Message parsing (parallel, mostly VCS independent)
- Change parsing (parallel, highly VCS dependent)

This implements commit discovery for Mercurial, similar to git's parsing:

- List the heads of all the branches.
- If we haven't already discovered them, follow them back to their roots (or
the first commit we have discovered).
- Import all the newly discovered commits…