make git_clone safer

the ensures that if the function returns early, we return to a
sane directory, and not hang out somewhere that a future git
call might modify a directory in a weird way. This is especially
important in the case of stable branches where were are hopping
between stable for servers and master for clients.

Change-Id: Ib8ebbc23b1813bc1bfb31d0a079f1b882135bd39
Merge "make git_clone safer"
Merge branch 'master' of github.com:openstack-dev/devstack