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.
workingfilectx.date = (1000, 0)
ASCII : Gr?ezi!
Latin-1 : Grüezi!
UTF-8 : Grüezi!
(['foo'], [], [], [], [], [], [])
diff --git a/foo b/foo
--- a/foo
+++ b/foo
@@ -1,1 +1,2 @@
foo
+bar