dirstate-tree: Make `DirstateMap` borrow from a bytes buffer … that has the contents of the `.hg/dirstate` file. This only applies to the tree-based flavor of `DirstateMap`. For now only the entire `&[u8]` slice is stored, so this is not useful yet. Adding a lifetime parameter to the `DirstateMap` struct (in hg-core) makes Python bindings non-trivial because we keep that struct in a Python object that has a dynamic lifetime tied to Python’s reference-counting and GC. As long as we keep the `PyBytes` that owns the borrowed bytes buffer next to the borrowing struct, the buffer will live long enough for the borrows to stay valid. However this relationship cannot be expressed in safe Rust code in a way that would statisfy they borrow-checker. We use `unsafe` code to erase that lifetime parameter, and encapsulate it in a safe abstraction similar to the owning-ref crate: https://docs.rs/owning_ref/ Differential Revision: https://phab.mercurial-scm.org/D10557