Fix an issue in arc patch with git-svn

Summary:
We recently moved our HEAD and it caused some issues on `arc patch` with git-svn repos. The base revision is incorrect and patch will fail. Add the check in such case to make it work.
The check was there before but removed in change b202158. The reason wasn't mentioned there though.

Test Plan: Tried it on svn.

Reviewers: lifeihuang, JoelB, #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: epriestley, Korvin

Differential Revision: https://secure.phabricator.com/D8717