t7508: don't use test_must_fail test_cmp

The test_must_fail function should only be used for git commands since
we 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 <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: Denton-L

t/t7508-status.sh

@@ -1471,7 +1471,7 @@ test_expect_success '"status.branch=true" same as "-b"' '
 test_expect_success '"status.branch=true" different from "--no-branch"' '
 	git status -s --no-branch  >expected_nobranch &&
 	git -c status.branch=true status -s >actual &&
-	test_must_fail test_cmp expected_nobranch actual
+	! test_cmp expected_nobranch actual
 '
 
 test_expect_success '"status.branch=true" weaker than "--no-branch"' '