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 commist import with "at lower priority" message and priority 4000 ("PRIORITY_IMPORT").

Maniphest Tasks: T13369

Differential Revision: https://secure.phabricator.com/D20712