Add a comment explaining the extra comparison in raw_hash_set::operator==. Also add a small optimization to avoid the extra comparison in sets that use hash_default_eq as the key_equal functor.

Note that removing the comparison entirely causes the Table.Equality2 test case to fail.

PiperOrigin-RevId: 693447075
Change-Id: I769f75cf1cb27114455f1854e330c899cee55fc2