Allow Phabricator to parse bare revision IDs from "Differential Revision:" fields

Summary: Fixes T8087. Depends on D12748.

Test Plan: See D12748.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: cburroughs, epriestley

Maniphest Tasks: T8087

Differential Revision: https://secure.phabricator.com/D12749