Don't allow git-describe failures to go unnoticed in t6120

If git-describe fails we never execute the test_expect_success,
so we never actually test for failure.  This is horribly wrong.
We need to always run the test case, but the test case is only
supposed to succeed if the prior git-describe returned 0.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6120-describe.sh

@@ -15,8 +15,10 @@ test_description='test describe
 check_describe () {
 	expect="$1"
 	shift
-	R=$(git describe "$@") &&
+	R=$(git describe "$@")
+	S=$?
 	test_expect_success "describe $*" '
+	test $S = 0 &&
 	case "$R" in
 	$expect)	echo happy ;;
 	*)	echo "Oops - $R is not $expect";