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

dirstate-tree: Add parsing only dirstate parents from disk Differential Revision: https://phab.mercurial-scm.org/D10368

dirstate-tree: Implement DirstateMap::read This reads the on-disk dirstate in its current format (a flat sequence of entries) and creates a tree in memory. Differential Revision: https://phab.mercurial-scm.org/D10367