Make --auto creates actually work in SVN and HG

Summary:
--auto doesn't work right now on the implicit --create pathway in SVN and HG because we hit these conditions.

Also improve a message.

Test Plan: Ran "arc diff" in unaffiliated working copies in HG and SVN.

Reviewers: svemir, btrahan, vrana, jungejason

Reviewed By: svemir

CC: aran

Differential Revision: https://secure.phabricator.com/D2187