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, oldest first.

This is a little complicated but it ensures we discover commits in depth order,
so the discovery process is robust against interruption/failure. If we just
inserted commits as we went, we might read the tip, insert it, and then crash.
When we ran again, we'd think we had already discovered commits older than HEAD.

This also allows later stages to rely on being able to find Phabricator commit
IDs which correspond to parent commits.

Test Plan: Discovered all the commits in the main Mercurial repository,
http://selenic.com/repo/hg.

Reviewers: Makinde, jungejason, nh, tuomaspelkonen, aran

Reviewed By: Makinde

CC: aran, Makinde

Differential Revision: 943