generatorset: explicitly track iteration order The expected iteration order may be different than the fast iteration order (eg: ancestors(42) is expected to be iterated upward but is fast/lazy to compute downward. So we explicitly track the iteration order and enforce it if the manual iteration is requested. Default expected iteration order of a generator set is ascending because I'm not aware of any descending revset that need a generatorset. The first to find such descending revset will have the pleasure to make this configurable.