Don't amend existing revision with `arc diff --create`

Summary: If there's other revision in last commit message and I said `--create` then it is clear that I want to create a new commit.

Test Plan:
  $ arc diff --create # in dirty working copy on top of my open revision

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5078