tests: make test_must_fail more verbose

Because test_must_fail fails when a command succeeds, the
command frequently does not produce any output (since, after
all, it thought it was succeeding). So let's have
test_must_fail itself report that a problem occurred.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-lib.sh

@@ -591,7 +591,15 @@ test_path_is_missing () {
 
 test_must_fail () {
 	"$@"
-	test $? -gt 0 -a $? -le 129 -o $? -gt 192
+	exit_code=$?
+	if test $exit_code = 0; then
+		echo >&2 "test_must_fail: command succeeded: $*"
+		return 1
+	elif test $exit_code -gt 129 -a $exit_code -le 192; then
+		echo >&2 "test_must_fail: died by signal: $*"
+		return 1
+	fi
+	return 0
 }
 
 # Similar to test_must_fail, but tolerates success, too.  This is