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.
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state