t5801: teach compare_refs() to accept !

Before, testing if two refs weren't equal with compare_refs() was done
with `test_must_fail compare_refs`. This was wrong for two reasons.
First, test_must_fail should only be used on git commands. Second,
negating the error code is a little heavy-handed since in the case where
one of the git invocations within compare_refs() fails, we will report
success, even though it failed at an unexpected point.

Teach compare_refs() to accept `!` as the first argument which would
_only_ negate the test_cmp()'s return code.

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

t/t5801-remote-helpers.sh

@@ -11,9 +11,15 @@ test_description='Test remote-helper import and export commands'
 PATH="$TEST_DIRECTORY/t5801:$PATH"
 
 compare_refs() {
+	fail= &&
+	if test "x$1" = 'x!'
+	then
+		fail='!' &&
+		shift
+	fi &&
 	git --git-dir="$1/.git" rev-parse --verify $2 >expect &&
 	git --git-dir="$3/.git" rev-parse --verify $4 >actual &&
-	test_cmp expect actual
+	eval $fail test_cmp expect actual
 }
 
 test_expect_success 'setup repository' '
@@ -189,7 +195,7 @@ test_expect_success GPG 'push signed tag' '
 	git push origin signed-tag
 	) &&
 	compare_refs local signed-tag^{} server signed-tag^{} &&
-	test_must_fail compare_refs local signed-tag server signed-tag
+	compare_refs ! local signed-tag server signed-tag
 '
 
 test_expect_success GPG 'push signed tag with signed-tags capability' '