Basic Mercurial support for Arcanist

Summary:
There's a lot of ground left to cover but this makes "arc diff" work (on one
trivial diff) in my sandbox, at least, and supports parsing of Mercurial native
diffs (which are unified + a custom header). Piles of missing features, still.
Some of this is blocked by me not understanding the mercurial model well yet.

This is also a really good opportunity for cleanup (especially, reducing the
level of "instanceof" in the diff workflow), I'll try to do a bunch of that in
followup diffs.

Test Plan: Ran "arc diff" in a mercurial repository, got a diff out of it.
Reviewed By: aran
Reviewers: Makinde, jungejason, tuomaspelkonen, aran, codeblock
CC: aran, epriestley, codeblock, fratrik
Differential Revision: 792