t4013: support test_expect_failure through ':failure' magic

Add support to be able to specify expected failure, through :failure
magic, like this:

:failure cmd args

Signed-off-by: Sergey Organov <sorganov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4013-diff-various.sh

@@ -174,6 +174,7 @@ process_diffs () {
 V=$(git version | sed -e 's/^git version //' -e 's/\./\\./g')
 while read magic cmd
 do
+	status=success
 	case "$magic" in
 	'' | '#'*)
 		continue ;;
@@ -182,6 +183,10 @@ do
 		label="$magic-$cmd"
 		case "$magic" in
 		noellipses) ;;
+		failure)
+			status=failure
+			magic=
+			label="$cmd" ;;
 		*)
 			BUG "unknown magic $magic" ;;
 		esac ;;
@@ -194,7 +199,7 @@ do
 	expect="$TEST_DIRECTORY/t4013/diff.$test"
 	actual="$pfx-diff.$test"
 
-	test_expect_success "git $cmd # magic is ${magic:-(not used)}" '
+	test_expect_$status "git $cmd # magic is ${magic:-(not used)}" '
 		{
 			echo "$ git $cmd"
 			case "$magic" in