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

modules/jenkins/files/slave_scripts/gerrit-git-prep.sh

@@ -29,6 +29,8 @@ fi
 set -x
 if [[ ! -e .git ]]
 then
+    ls -a
+    rm -fr .[^.]* *
     git clone $GERRIT_SITE/p/$ZUUL_PROJECT .
 fi
 git remote set-url origin $GERRIT_SITE/p/$ZUUL_PROJECT