dirstate-tree: Change status() results to not borrow DirstateMap The `status` function takes a `&'tree mut DirstateMap<'on_disk>` parameter. `'on_disk` borrows a read-only byte buffer with the contents of the `.hg/dirstate` file. `DirstateMap` internally uses represents file paths as `std::borrow::Cow<'on_disk, HgPath>`, which borrows the byte buffer when possible and allocates an owned string if not, such as for files added to the dirstate after it was loaded from disk. Previously the return type of of `status` has a `'tree` lifetime, meaning it could borrow all paths from the `DirstateMap`. With this changeset, that lifetime is changed to `'on_disk` meaning that only paths from the byte buffer can be borrowed, and paths allocated by `DirstateMap` must be copied. Usually most paths are in the byte buffer, and most paths are not part of the return value of `status`, so the number of extra copies should be small. This change will enable `status` to mutate the `DirstateMap` after it has finished constructing its return value. Previously such mutation would be prevented by possible on-going borrows. Differential Revision: https://phab.mercurial-scm.org/D10824