Parse VCS revert langauge in commit messages

Summary:
Ref T1751. When commit messages include language like "reverts X", parse it. This change doesn't do anything with the commits yet.

I attempted to cover all "natural" VCS messages and all reasonable human variations of these messages.

Test Plan: Added unit tests. Added `var_dump()` and used `scripts/repository/reparse.php --message X` to reparse some commit messages, with expected results.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1751

Differential Revision: https://secure.phabricator.com/D5840