stablerange: drop the basic inheritance from the 'mergepoint' version All necessary bits are implemented now (except for warmup which we explicitly bypass for now).

stablerange: cache known subrange at the stablerangecached level That way, the 'stablerange_mergepoint' class also benefit from it.

stablerange: use sort cache to build 'mergepoint' information Using the walk cache gives use a better complexity we need to scale. There are further improvement we could make, but this is a good first step.

stablesort: make the iteration from head available to all This will be useful for the stablerange code to work properly.

depth: fix compare mode for `debugdepth` The compare mode was broken for silly reasons.

memfilectx: changectx argument is not mandatory As changectx argument is now mandatory for memfilectx, add a compatibility layer to pass it when necessary. The commit that made the parament mandatory in core is 8a0cac20a1ad

branching: merge stable into default