improve git code with devstack functions

improve git code with devstack functions

if we aren't re-cloning we still need to make sure
we update the branch/tag/commit to the version from the remote

so we need to fetch and then checkout again

it is more complicated.  Dockerfile does a shallow
clone, so we don't get all the remote ref information.

To clone a random remote commit id we have to unshallow

Change-Id: If15f504b6ff2cfdfc8894fd3b37e687a19616714
Signed-off-by: Ross Brattain <ross.b.brattain@intel.com>