Quit early if candidate already exists; invoke buildbot if changes found

Change-Id: I3d6294c12212967f9b7a094ecc2ce90ea22f5f2f
Reviewed-on: http://review.couchbase.org/33449
Reviewed-by: Chris Hillery <ceej@couchbase.com>
Tested-by: Chris Hillery <ceej@couchbase.com>