revset: make balanced addsets by orset() without using _combinesets() As scmutil.revrange() was rewritten to not use _combinesets(), we no longer need _combinesets().

revrange: evaluate all revset specs at once This provides an opportunity for revset to optimize given expressions. For example, "-r0 -r1 -r2" can be optimized to "_list(0 1 2)".