Have DifferentialRevisionList return an ObjectBox if header is set

Summary: Moving towards a consisent 'if header, show in object box' style around Phabricator.

Test Plan:
Grep for uses of RevisionList and make sure double boxes arent set, browse Differential, various searches, a revision, and a commit.

Reviewers: btrahan, epriestley

Reviewed By: epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D11651