Add a proper option to enable branch auto-detection.

Also, remove the "$default" param from our Git RepoManager because with
this better support for auto-detection, we no longer have a clear use
for it.
Remove the 'logout' call to Jenkins.

At Etsy, we now use SSH auth with Jenkins instead of password auth. So
we don't have to worry about a lingering logged-in session for our users
-- either their SSH keys are still there or they aren't. Thus the
'logout' command is just wasted time now.
1.2.0 release