Always provide a viewer when executing DifferentialRevisionQuery

Summary: Ref T603. This query isn't policy-aware yet, but prepare for it to be one day.

Test Plan: Looked at: home page; differential home; differential detail; diffusion browse. Made differential.query conduit call.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T603

Differential Revision: https://secure.phabricator.com/D6337