t5801: teach compare_refs() to accept !

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>