fix bin/audit to not query whole DB if no commits found

Summary: reported by csilvers in irc

Test Plan: ran a bum query with --trace and verified table scan not run

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, aran

Differential Revision: https://secure.phabricator.com/D6986
Merge branch 'master' of http://github.com/facebook/phabricator