dirstate-tree: Fold "tracked descendants" counter update in main walk For the purpose of implementing `has_tracked_dir` (which means "has tracked descendants) without an expensive sub-tree traversal, we maintaing a counter of tracked descendants on each "directory" node of the tree-shaped dirstate. Before this changeset, mutating or inserting a node at a given path would involve: * Walking the tree from root through ancestors to find the node or the spot where to insert it * Looking at the previous node if any to decide what counter update is needed * Performing any node mutation * Walking the tree *again* to update counters in ancestor nodes When profiling `hg status` on a large repo, this second walk takes times while loading a the dirstate from disk. It turns out we have enough information to decide before he first tree walk what counter update is needed. This changeset merges the two walks, gaining ~10% of the total time for `hg update` (in the same hyperfine benchmark as the previous changeset). --- Profiling was done by compiling with this `.cargo/config`: [profile.release] debug = true then running with: py-spy record -r 500 -n -o /tmp/hg.json --format speedscope -- \ ./hg status -R $REPO --config experimental.dirstate-tree.in-memory=1 then visualizing the recorded JSON file in https://www.speedscope.app/ Differential Revision: https://phab.mercurial-scm.org/D10554