Policy - clean up the deprecated diffusion.getcommits

Summary: Ref T7094. Could just delete this end point too I guess? Needed to add "withCommitPHIDs" to the differentialrevisionquery to get this done.

Test Plan: used diffusion.getcommits from conduit console and got a sensible result for a query for two commits, one with a diff and one without.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7094

Differential Revision: https://secure.phabricator.com/D11581