t5512: don't use `test_must_fail test_cmp`

The test_must_fail function should only be used for git commands since
we should assume that external commands work sanely. Since test_cmp() just
wraps an external command, replace `test_must_fail test_cmp` with
`! test_cmp`.

Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t5512-ls-remote.sh

@@ -92,7 +92,7 @@ test_expect_success 'use "origin" when no remote specified' '
 
 test_expect_success 'suppress "From <url>" with -q' '
 	git ls-remote -q 2>actual_err &&
-	test_must_fail test_cmp exp_err actual_err
+	! test_cmp exp_err actual_err
 '
 
 test_expect_success 'use branch.<name>.remote if possible' '