revset: added reverse method to addset This method is needed to duck type generatorset.

revset: changed _iterator() method on addset to work with a given order If the two collections are in ascending order, yield their values in an ordered way by iterating both at the same time and picking the values to yield.

revset: changed _iterator() in addset to use the generated list when available Now when all the elements have been generated, the iterator will just use the generated list instead of going through all the elements again.