tasks/workunit.py: specify the branch name when cloning a branch

tasks/workunit.py: specify the branch name when cloning a branch

c1309fb failed to specify a branch when cloning using --depth=1, which
by default clones the HEAD. and we can not "git checkout" a specific
sha1 if it is not HEAD, after cloning using '--depth=1', so in this
change, we dispatch "tag", "branch", "HEAD" using three Refspec classes.

Signed-off-by: Kefu Chai <kchai@redhat.com>
Signed-off-by: Dan Mick <dan.mick@redhat.com>
(cherry picked from commit 9ca7ccf5f1739f731da8bf31260594aea3a2932d)

Conflicts:
	qa/tasks/workunit.py (trivial resolution)