R1:3cb543ef8fc6

Lift logic for queueing commit import tasks into RepositoryEngine

Summary:
Ref T13591. There are currently two pathways to queue an import task for a commit: via repository discovery, or via a ref becoming permanent.

These pathways duplicate some logic and have behavioral differences: one does not set `objectPHID` properly, one does not set the priority correctly.

Unify these pathways, make them both set `objectPHID`, and make them both use the same priority logic.

Test Plan:
- Discovered refs.