subranges: add a utility function to access the cache This is preparing on disk persistence for the value in this cache.

depth: add a utility function to set the cache This is preparing on disk persistence for the value in this cache.

depth: add a utility function to access the cache This is preparing on disk persistence for the value in this cache.

stablerange: add warming of the subrange Note that this means we build standard stable subrange for all changesets in the repository this is significantly more than what we were computing before and result is significantly more ranges being computed.

stablerange: fix merge slicing when range has multiple roots The first element in the bottom set is not necessarly the one with the lowest revision. We now properly compute and use the minimum value.

stablerange: small style fix

merge-slicing: introduce and use "inheritance point" for merge The first part of the stable sorted list of revision of a merge will shared with the one of others. This means we can reuse subranges computed from that point to compute some of the subranges from the merge. That point is latest point in the stable sorted list where the depth of the revisions match its index (that means all revision earlier in the stable sorted list are its ancestors, no dangling unrelated branches exists). This is a bit expensive to find since we have to walk all the revision, but being able to reuse subranges in all case (not just regular changesets) provide a massive speedup so the cost is worth it.

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.