debugdeltachain: stop summing the same chain over and over Before this patch, delta chain size was computed from scratch for each chain, disregarding the fact very likely already computed the same of length-1 prefix for another revisions. We not cache delta chain size and shortcut the computation when we see them. Just for my mercurial-devel clone, this move the computation from about 17.5 second to about 4.8 seconds.