Add a differential.getdiffs method

Summary: I kind of made a mess of the API doing T2784. I figure just adding this is fine but LMK if you'd prefer something like diffquery got cleaned up more to handle this. Also adds an idx() call as I was getting errors looking at old diffs. Fixes T3823.

Test Plan: used the new api via test console - great success.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Maniphest Tasks: T3823

Differential Revision: https://secure.phabricator.com/D6966