revlog: add a doctest to _trimchunk

revlog: introduce a tiny mock of a revlog class This will be used in various function doctest added in the next changesets. The class seems small enough to live in the module directly.

revlog: _segmentspan computes the byte span of a segment Refactor out this code to be used somewhere else in a next changeset

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.