Policy - lock down ReleephCommitFinder

Summary: Not too shabby - just convert some raw queries to the policy queries. Ref T7094.

Test Plan: NA 'cuz releeph

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7094

Differential Revision: https://secure.phabricator.com/D11591