Clear workspace before cloning in GGP.

* modules/jenkins/files/slave_scripts/gerrit-git-prep.sh: The
git-clone subcommand fails spectacularly when its destination is set
to a non-empty directory. In the case of gerrit-git-prep this can
happen when files are left behind in a workspace with no .git tree,
and then the job is changed to run GGP later. Work around it by
forcibly removing any previous workspace contents before cloning,
but show a listing of any contents first for the benefit of
troubleshooting such situations.

Change-Id: I0662ee68ee2a0e7a66a4d85c8cc7aea8bf0c17c0
Merge "Clear workspace before cloning in GGP."