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

dirstate-tree: Add clear_ambiguous_times in the new DirstateMap Also drive-by refactor it in the other DirstateMap Differential Revision: https://phab.mercurial-scm.org/D10489

dirstate-tree: Add copy_map_insert and copy_map_remove Differential Revision: https://phab.mercurial-scm.org/D10488

dirstate-tree: Maintain a counter of DirstateEntry’s and copy sources This allows implementing __len__ for DirstateMap and CopyMap efficiently, without traversing the tree. Differential Revision: https://phab.mercurial-scm.org/D10487

dirstate-tree: Serialize to disk The existing `pack_dirstate` function relies on implementation details of `DirstateMap`, so extract some parts of it as separate functions for us in the tree-based `DirstateMap`. The `bytes-cast` crate is updated to a version that has an `as_bytes` method, not just `from_bytes`: https://docs.rs/bytes-cast/0.2.0/bytes_cast/trait.BytesCast.html#method.as_bytes Drive-by refactor `clear_ambiguous_times` which does part of the same thing. Differential Revision: https://phab.mercurial-scm.org/D10486

rust: Add a Timestamp struct instead of abusing Duration `SystemTime` would be the standard library type semantically appropriate instead of `Duration`. But since the value is coming from Python as a plain integer and used in dirstate packing code as an integer, let’s make a type that contains a single integer instead of using one with sub-second precision. Differential Revision: https://phab.mercurial-scm.org/D10485

dirstate-tree: Add tree traversal/iteration Like Python’s, Rust’s iterators are "external" in that they are driven by a caller who calls a `next` method. This is as opposed to "internal" iterators who drive themselves and call a callback for each item. Writing an internal iterator traversing a tree is easy with recursion, but internal iterators cannot rely on the call stack in that way, they must save in an explicit object all state that they need to be preserved across two `next` calls. This algorithm uses a `Vec` as a stack that contains what would be local variables on the call stack if we could use recursion. Differential Revision: https://phab.mercurial-scm.org/D10370

dirstate-tree: Add map `get` and `contains_key` methods Differential Revision: https://phab.mercurial-scm.org/D10369