test-import.t: use proper revset order This test, written after 3.0, is relying on addset being enforced ascending if both side are ascending. We are about to restore the ordering to 2.9 behavior (elements are ordered in the order they are specified). We fix the test before fixing the order.

baseset: drop custom __sub__ method This add method is enforcing non-laziness, disabling multiple optimisations. Benchmarks do not spot any significant difference but real usecase may. This will also be important for further improvements to addset later in this series.