Add branch support to preflight
Add branch support to preflight

Change-Id: Ia2592c6384e94e4e8e5400eddbf2e03aac150eef

BUG=7905
TEST=run cbuildbot on my workstation.

Review URL: http://codereview.chromium.org/4040002

trnubo/scripts,gtank/scripts,coreos/scripts,crawford/scripts,crawford/scripts,BugRoger/scripts,bcwaldon/coreos-scripts,marineam/coreos-scripts,mischief/scripts,nicescale/scripts,marineam/coreos-scripts,coreos/scripts,glevand/coreos--scripts,zhang0137/scripts,andrejro/coreos--scripts,vmware/coreos-scripts,trnubo/scripts,zhang0137/scripts,endocode/scripts,smilart/scripts,mjg59/scripts,trnubo/scripts,glevand/coreos--scripts,vmware/coreos-scripts,endocode/scripts,sigma/coreos-scripts,gtank/scripts,BugRoger/scripts,mischief/scripts,gtank/scripts,mjg59/scripts,marineam/coreos-scripts,mjg59/scripts,endocode/scripts,vmware/coreos-scripts,bcwaldon/coreos-scripts,fivethreeo/scripts,crawford/scripts,andrejro/coreos--scripts,fivethreeo/scripts,BugRoger/scripts,mischief/scripts,nicescale/scripts,nicescale/scripts,sigma/coreos-scripts,smilart/scripts,bcwaldon/coreos-scripts,sigma/coreos-scripts,zhang0137/scripts,smilart/scripts,fivethreeo/scripts,andrejro/coreos--scripts