Reduce parse latency for changes pushed to hosted repositories

Summary:
Currently, we can sit in a sleep() for up to 15 seconds (or longer, in some cases), even if there's a parse request.

Try polling the DB every second to see if we should wake up instead. This might or might not produce significant improvements, but seems OK to try and see.

Also a small fix for logging branch names with % in them, etc.

Test Plan: Ran phd debug pulllocal and pushed commits, saw them parse within a second.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, dctrwatson

Differential Revision: https://secure.phabricator.com/D7998