addset: implement first and last methods The implementation is non-lazy for now. One may want to make it more lazy in the future.

spanset: implement `first` and `last` methods

smartset: add first and last methods In multiple places in the code, we use `someset[0]` or `someset[-1]`. This works only because the `someset` is usually a baseset. For the same reason we introduce a `first` and `last` methods to be implemented for all smartset classes.

getgraphlogrevs: remove user of baseset.append A `baseset` has multiple cached results and will get even more in the future. Making it an object "populated once" like the other smartsets makes it both safer and simpler. The append method will be removed at some point.

getlogrevs: remove user of baseset.append A `baseset` has multiple cached results and will get even more in the future. Making it an object "populated once" like the other smartsets makes it both safer and simpler. The append method will be removed at some point.

revset-last: remove user of baseset.append A `baseset` has multiple cached results and will get even more in the future. Making it an object "populated once" like the other smartsets makes it both safer and simpler. The append method will be removed at some point.

revset-limit: remove user of baseset.append A `baseset` has multiple cached results and will get even more in the future. Making it an object "populated once" like the other smartsets makes it both safer and simpler. The append method will be removed at some point.

mq: use `revs.sort()` to ensure the set is ascending Sorting is super-cheap with the new smartset class, so we can use it to enforce the order. Otherwise all smartset classes would have to allow direct indexing.