revpair: use `first` and `last` instead of direct indexing This makes it compatible with all smarsets classes.

revsingle: use `last` instead of direct indexing This makes it compatible with all smarsets classes.

revset-limit: use boolean testing instead of `len(revs) < 1` I'm not sure why we wrote it that way. But smartsets have faster/lazier non-zero testing than length computation.

filteredset: implement `first` and `last`

baseset: implement `first` and `last` methods

generatorset: implement first and last methods

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.