Arcanist / Conduit - stop using deprecated differential.getdiff API

Summary: Fixes T7112. Nothing too difficult here.

Test Plan:
meta - submitting this with the new arcanist code
used conduit API to verify the difference between getdiff (just the latest diff) and querydiffs (all diffs that match, with the latest diff first in the set)

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Maniphest Tasks: T7112

Differential Revision: https://secure.phabricator.com/D11665