Arcanist / Conduit - stop using deprecated diffusion.getcommits

Summary: Fixes T7113. This one was a bit trickier than others as the API output changed a bit. In particular, there is no "errors" emitted so much as the result set just doesn't include the answer if things are garbage. Ergo, check the "identifier map" to either check for diff existence or to lookup the phid to grab the actual diff data from the "data" part of the result.

Test Plan: called arc backout D11665 and got some working output...!

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7113

Differential Revision: https://secure.phabricator.com/D11667