R1:cd71098110e2

Detect commits by hash relationships

Summary:
When we discover a new commit and it has a known local commit or tree hash, mark
it committed.

This supports Mercurial and Git-Immutable workflows, and improves
hybrid-Git-Mutable workflows and covers some cases where poeple just make
mistakes or whatever.

Test Plan: Parsed Mercurial, Git and SVN commits.

Reviewers: Makinde

Reviewed By: Makinde

CC: aran, Makinde

Differential Revision: 963