R1:82cf97ad65a9

When many commits are discovered at once, import them at lower priority

Summary:
Ref T13369. See that task for discussion.

When the discovery daemon finds more than 64 commits to import, demote the worker queue priority of the resulting tasks.

Test Plan:
- Pushed one commit, ran `bin/repository discover --verbose --trace ...`, saw commit import with "at normal priority" message and priority 2500 ("PRIORITY_COMMIT").
- Pushed 3 commits, set threshold to 3, ran `bin/repository discover ...`, saw…