Add pruning in validation cloning

The gate keeps a local cache of repos that will not reflect things like
the manual deletion of refs such as incorrect branches. In order for the
validation to pass before the cache is rebuilt, we need to prune the
repo to pick up those deletions.

Change-Id: I25848275f90ec440e85765f87a23cd1413aab6cb
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

tools/clone_repo.sh

@@ -173,7 +173,7 @@ fi
 
 # Make sure it is up to date compared to the upstream remote.
 (cd $local_dir &&
-        git fetch origin --tags
+        git fetch origin --tags --prune
 )
 
 if [ ! -z "$REF" ]; then