Add unit tests for the Mercurial change parser

Summary: Ref T4327. This adds a set of test cases for the Mercurial parser. These tests are nearly identical to the git cases, except that the Mercurial parser can't figure out symlinks right now.

Test Plan: Ran unit tests.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4327

Differential Revision: https://secure.phabricator.com/D8014