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.
requesting all changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 4 changes to 4 files
updating to branch default
4 files updated, 0 files merged, 0 files removed, 0 files unresolved
requesting all changes
adding changesets
adding manifests
adding file changes
added 1 changesets with 4 changes to 4 files
updating to branch default
4 files updated, 0 files merged, 0 files removed, 0 files unresolved