Move Differential commit message parsing to a separate, tested class

Summary: Ref T2222. We have a hunk of logic that purely does text parsing here; separate it and get coverage on it.

Test Plan:

• Ran new unit tests.
• Used differential.parsecommitmessage.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8444