t3900: test rejecting log message with NULs correctly

It is not like that our longer term desire is to someday start
accept log messages with NULs in them, so it is wrong to mark a test
that demonstrates "git commit" that correctly fails given such an
input as "expect-failure".  "git commit" should fail today, and it
should fail the same way in the future given a message with NUL in it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t3900-i18n-commit.sh

@@ -34,9 +34,9 @@ test_expect_success 'no encoding header for base case' '
 	test z = "z$E"
 '
 
-test_expect_failure 'UTF-16 refused because of NULs' '
+test_expect_success 'UTF-16 refused because of NULs' '
 	echo UTF-16 >F &&
-	git commit -a -F "$TEST_DIRECTORY"/t3900/UTF-16.txt
+	test_must_fail git commit -a -F "$TEST_DIRECTORY"/t3900/UTF-16.txt
 '