stablerange: compute subranges from parent when possible Unless we are a merge, we can infer the standard subranges of a range by reusing the subranger of its parent. We update the implementation to do so. We still needs the "old way" for merge. We move that code in a dedicated function for the sake of simplicity.

stablerange: add an official warmup function The function is responsible for making sure we have the necessary data for a sets of heads. For now this only warm the depth cache. More will follow soon. Such explicite warmup will be usefull for on disck caching as an early point of writing will be easier.