did not checkout to new branch if repo already exist

if localrc has defined branches for the components
and the git repo already exist
stack.sh will bypass cloning repo altogether
when it should checkout to new branch as defined in localrc

step to reproduce:
1. run devstack.sh till completion, when all repo is cloned
2. re-run devstack.sh with lines below added to localrc

GLANCE_BRANCH=essex-3
HORIZON_BRANCH=essex-3
KEYSTONE_BRANCH=essex-3
KEYSTONECLIENT_BRANCH=essex-3
NOVA_BRANCH=essex-3
NOVACLIENT_BRANCH=essex-3
QUANTUM_BRANCH=essex-3
QUANTUM_CLIENT_BRANCH=essex-3

3. the git repo is not checkout to the defined branch