Create a new diff if the user says so

Summary: If I have //Differential Revision// in my commit message then `arc diff --create` updates that revision instead of creating a new one.

Test Plan:
  $ arc diff --create # on top of commit message with Differential Revision

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5077