Releeph introduces more partial-commit-identifier loading code, in ReleephCommitFinder. We should consolidate all the copies of this code:

• One copy in ConduitAPI_diffusion_getcommits_Method.
• One copy in DiffusionCommitQuery.
• Part of one copy in PhabricatorAuditCommitQuery.
• One copy in PhabricatorPHID::fromObjectName().
• A new copy in ReleephCommitFinder.

Of these, DiffusionCommitQuery is the most modern (includes policy awareness and handles all the nasty edge cases).