[diffusion] Fix pending differential revisions list

Summary:
DifferentialRevisionListView requires setFields to be called before
calling getRequiredHandlePHIDs; this adds that call for DiffusionController

Test Plan:
loaded diffusion and saw the "Pending Differential Revisions" section
populated, and no errors in the darkconsole

Reviewers: epriestley, btrahan, jungejason

Reviewed By: epriestley

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1665