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.