R1:faf3f7b7872a

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…