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.
Minimal hgk check
$ echo "[extensions]" >> $HGRCPATH
$ echo "hgk=" >> $HGRCPATH
$ hg init repo
$ cd repo
$ echo a > a
$ hg ci -Am adda
adding a
$ hg debug-cat-file commit 0
tree a0c8bcbbb45c
parent 000000000000
author test 0 0
revision 0
branch default
phase draft
adda
$ cd ..