dirstate-tree: Keep a counter of descendant nodes that have an entry … and change the `DirstateMap::has_dir` method to be based on this counter being non-zero instead of the mere presence of a node. A node with zero descendent with an entry currently should be removed from the tree, but soon we’ll make the dirstate track additional nodes. (Specifically, for non-ignored directories in order to keep track of their mtime and optimize status by doing fewer `read_dir` calls.) Differential Revision: https://phab.mercurial-scm.org/D10922