Add a Mercurial message parser

Summary: See D943, this is the second parse stage. This will mark Differential revisions as "Committed" among other things.

Almost all the logic here is shared between VCSes so the implementation itself is straightforward.

Test Plan: Parsed all messages for the official Mercurial repository.

Reviewers: Makinde, jungejason, nh, tuomaspelkonen, aran

CC:

Differential Revision: 944