cros_mark_as_stable: delete merge_branch if it already exists
cros_mark_as_stable: delete merge_branch if it already exists

Fixes issue with pfbb where commits aren't getting squashed

BUG=6102
TEST=Ran the code.

Change-Id: Ibbcb8882ee13abf3a7c283bb0d248eef8db31cb8

Review URL: http://codereview.chromium.org/3141045

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