Make sources-branch-updater checkout the SHA properly
Make sources-branch-updater checkout the SHA properly

Currently the sources-branch-updater script incorrectly handles the
clone/checkout process and ends up always copying the master branch
files into whichever branch it's trying to update.

This fixes it so that it correctly checks out to the correct SHA
before copying the updated files over.

Change-Id: I684bc060bf0a4bf4398673b4ca84a2660ee6e1b9

BjoernT/os-ansible-deployment,stackforge/os-ansible-deployment,cloudnull/os-ansible-deployment,os-cloud/os-ansible-deployment,openstack/openstack-ansible,cfarquhar/openstack-ansible,major/openstack-ansible,yanyao/openstack-ansible,BjoernT/os-ansible-deployment,mrda/openstack-ansible,yanyao/newton,robb-romans/openstack-ansible,robb-romans/openstack-ansible,git-harry/os-ansible-deployment,BjoernT/openstack-ansible,git-harry/os-ansible-deployment,major/os-ansible-deployment,cfarquhar/openstack-ansible,Logan2211/openstack-ansible,yanyao/newton,mancdaz/openstack-ansible,weezer/openstack-ansible,yanyao/openstack-ansible,Logan2211/openstack-ansible,mrda/openstack-ansible,cloudnull/os-ansible-deployment,mancdaz/openstack-ansible,stackforge/os-ansible-deployment,bunchc/openstack-ansible,major/openstack-ansible,git-harry/os-ansible-deployment,BjoernT/openstack-ansible,bunchc/openstack-ansible,major/os-ansible-deployment,weezer/openstack-ansible,openstack/openstack-ansible,mrda/openstack-ansible,BjoernT/os-ansible-deployment,os-cloud/os-ansible-deployment