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.

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