scripts: sources-branch-updater-lib.sh: Avoid using 'dirname $0'
scripts: sources-branch-updater-lib.sh: Avoid using 'dirname $0'

Since this library can be sourced by external scripts, 'dirname $0'
can't be used reliably to determine where the rest of the OSA helper
scripts are located since $0 points to the external script.
It's best to use the BASH_SOURCE array to determine the file in which
the current function is defined.

Change-Id: If48ee9cb472de811076d7d3adec8f3c9b476b833

stackforge/os-ansible-deployment,cloudnull/os-ansible-deployment,yanyao/openstack-ansible,bunchc/openstack-ansible,openstack/openstack-ansible,stackforge/os-ansible-deployment,yanyao/openstack-ansible,bunchc/openstack-ansible,cfarquhar/openstack-ansible,openstack/openstack-ansible,cloudnull/os-ansible-deployment,cfarquhar/openstack-ansible