Fix 'arc lint --everything' when svn has uncommitted changes

Summary: My recent change adding --everything to arc lint could sometimes cause a "diff is empty" error, this patch fixes it.

Test Plan: Ran "arc lint --everything" before and after patch. No longer errors out. Only appeared to originally happen when there were uncommited changes in an svn repo.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Differential Revision: https://secure.phabricator.com/D6732