smartset: micro optimize baseset.slice() to use slice of list -1ms isn't significant, but seems not bad. revset #1: limit(0::9999, 100, 9000) 5) 0.001681 6) 0.000790

smartset: extract method to slice abstractsmartset Sub classes can provide optimized implementations.

smartset: extract spanset factory to make it constructed without a repo This renames the spanset class to _spanset, and moves its __init__ to new spanset() function. spanset() is now a factory function. This allows us to construct a spanset without keeping a repo instance.

smartset: change repr of spanset to show revisions as half-open range Before, an empty spanset was displayed as '<spanset+ 0:-1>', which seemed confusing.