revert: bring back usage of `subset & ps` in `parents` Changeset 95af98616aa7 switched the order of the operand of the "&" computation to work around an issue from repo-wide spanset. The need for a workaround has been alleviated by the introduction of `fullreposet`. So we restore it to normal. The benchmark shows no significant changes as expected. We also revert the bogus test change introduced by 95af98616aa7. The order is actually important.