suppress the 'no new changes found' output when polling for new changes

Summary:
This is mostly motivated by @csilver's suggestion a few weeks ago to
"show the full output of the last run [Mr] did":

https://khanacademy.hipchat.com/history/room/19528/2012/12/20

By suppressing those unnecessary "searching for changes... no changes found"
messages that go on every 15-seconds, we automatically show the last deploy
output. See screenshot:
https://s3.amazonaws.com/uploads.hipchat.com/22586/121261/zflfsd06ysiy3di/upload.png

Part two, which should be D1003, fixes the status (in this case "I'm happy") to
display correctly so "it's easy to see if mr is currently unhappy"

Test Plan:
Locally... Joel happened to push to stable at a convenient time while
I was testing things :)

Reviewers: alpert, benkomalo

Reviewed By: benkomalo

CC: csilvers

Differential Revision: http://phabricator.khanacademy.org/D1002