re-factor DifferentialChangesetParser pass 3 / N

Summary: introducing a new friend called DifferentialHunkParser. Sort of like the DifferentialChangesetParser but works with hunks only. tried to grab hunk parsing type things from across the code base and move them into this new class.

Test Plan: unit tests and played around in Differential a bit.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2009

Differential Revision: https://secure.phabricator.com/D4351