fetch: fix check for local ref path

This is a fast case if the slave has a local checkout available so that
it doesn't have to do a full clone. The check unfortunately didn't take
into account all cases and failed on the new slave.

(cherry picked from commit d867f3d470)

scripts/fetch.sh

@@ -40,11 +40,11 @@ update_repo() {
 		git fetch origin || die "unable to fetch ${uri}"
 	else
 		echo "Cloning '${path}'"
-		if [ -z "${GIT_LOCAL_REF_DIR}" ]; then
-			git clone ${uri} ${path} || die "unable to clone ${uri}"
-		else
+		if [ -d "${GIT_LOCAL_REF_DIR}" ]; then
 			git clone --reference ${GIT_LOCAL_REF_DIR}/`basename ${path}` \
-                ${uri} ${path} || die "unable to clone ${uri}"
+				${uri} ${path} || die "unable to clone ${uri}"
+		else
+			git clone ${uri} ${path} || die "unable to clone ${uri}"
 		fi
 		pushd ${path} > /dev/null
 	fi