Differential - change "closed by commit" comment to real transaction

Summary:
Implements a new transaction - still TYPE_ACTION - but using a new DifferentialAction::ACTION_COMMIT_CLOSE. Augment rendering as necessary to display this new transaction. Saves enough information so T3686 is possible but stops short of implementing a popup to display this information. Fixes T5875. Ref T3686.

One small display oddity - this new transaction now renders at the top of the transaction group whereas when it was a comment it was on the bottom. I think this is basically okay but if not how fix? (Playing with the "strength" of these actions will mess up the email too?)

Test Plan: made a diff X that fixed task Y. committed. checked diff X, task Y, and the commit pages for proper transactions and all looked good.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: epriestley, Korvin

Maniphest Tasks: T3686, T5875

Differential Revision: https://secure.phabricator.com/D10485