t5612: 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/t5612-clone-refspec.sh

@@ -87,7 +87,7 @@ test_expect_success 'by default no tags will be kept updated' '
 		git for-each-ref refs/tags >../actual
 	) &&
 	git for-each-ref refs/tags >expect &&
-	test_must_fail test_cmp expect actual &&
+	! test_cmp expect actual &&
 	test_line_count = 2 actual
 '