[devstack] Recent change breaks cloning with GIT_DEPTH setting

FYI, devstack commit b0bd5b92 doesn't work with a GIT_DEPTH=1 setting. That commit basically changed from git clone ... --branch <BRANCH> to git clone ... git checkout <BRANCH> So it's easy for the desired <BRANCH> to be unavailable if GIT_DEPTH is limited. Best wishes - Nell