Internally, align commit processing tasks around PHIDs, not IDs

Summary: Ref T13591. This is a minor consistency change to use PHIDs instead of IDs in the commit import processing pipeline. PHIDs are generally more powerful in more contexts and it would be unusual for a modern worker to use an ID here.

Test Plan:

• Made the "accept either ID or PHID" part of the change only.
• Pushed a commit, parsed and reparsed it step by step (this tests that "commitID" tasks can still process normally).
• Made the "write PHIDs" part of the change.
• Pushed a commit, parsed and reparsed it step by step.
• Looked at the task row in the database, saw PHID data.

Maniphest Tasks: T13591

Differential Revision: https://secure.phabricator.com/D21533