revlog: early return in _slicechunk when span is already small enough If the full span is smaller than the minimum gap size we'll consider, we know we won't do any slicing and we can return earlier.

revlog: early return in _slicechunk when density is already good We don't need to do anything if we know we won't enter the second while loop. Save the overhead of the first loop by returning earlier.

sparse-read: discard gap below 65K only Testing on actual data shows that 65K is more efficient in both time and memory than 256K.