revlog: add function to slice chunk down to a given size It is possible to encounter situations where the slicing based on density did not achieve chunk smaller than the 4*textlength limit. To avoid extra memory consumption in those cases, we need to be able to break down chunk to a given size. Actual caller comes in the next changesets.

revlog: extract density based slicing into its own function We are going to introduce another slicing step. We start by extracting the existing one into its own function.

revlog: document and test _slicechunk

revlog: add a doctest for _segmentspan

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.