dirstate-tree: Add has_dir and has_tracked_dir A node without a `DirstateMap` entry represents a directory. Only some values of `EntryState` represent tracked files. A directory is considered "tracked" if it contains any descendant file that is tracked. To avoid a sub-tree traversal in `has_tracked_dir` we add a counter for this. A boolean flag would become insufficent when we implement remove_file and drop_file. `add_file_node` is more general than needed here, in anticipation of adding the `add_file` and `remove_file` methods. Differential Revision: https://phab.mercurial-scm.org/D10490