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.
• See later changes in this series for more complete test cases.

Maniphest Tasks: T13591

Differential Revision: https://secure.phabricator.com/D21516