stablerange: rearrange the code picking subrange to warm Same logic, as the previous changesets, we prepare the code before adding merge support.

stablerange: rearrange the reusing logic to prepare to merge We'll soon be able to reuse some lower range when dealing with merge too. So we prepare the code for this in advance for clarity.

merge-slicing: explain an alternative implementation in a comments It has a better time complexity so a C implementation would likely out perform the current implementation

merge-slicing: use reachable roots to filter the various branches Reachable roots does what we want and have a quite fast C implementation.

merge-slicing: simplify various aspect of the code Especially the case were the bottom have single heads is not more efficiently handled.

stablerange: soon it will not provide any benefit and it gets in the way This was a bit strange and memory consuming anyway.