rhg: Add Repo::write_dirstate This method is not used yet. It saves to disk any mutation that was done to the `Repo`’s dirstate through `Repo::dirstate_map_mut`. It takes care of dirstate-v1 v.s. dirstate-v2, dockets, data files, appending when possible, etc. Differential Revision: https://phab.mercurial-scm.org/D11839

rhg: Add lazy/cached dirstate data file ID parsing on Repo The `dirstate_parents`, `dirstate_data_file_uuid`, and `dirstate_map` members of `Repo` can be access in any order and the `.hg/dirstate` file should only be opened once. Differential Revision: https://phab.mercurial-scm.org/D11838

rhg: Make Repo::dirstate_parents a LazyCell Unify with the same abstraction used for other lazily-initialized components Differential Revision: https://phab.mercurial-scm.org/D11837

rust: Add Vfs::write_atomic This method writes to a temporary file then renames in place Differential Revision: https://phab.mercurial-scm.org/D11836

rhg: Initial repository locking Initial Rust implementation of locking based on the `.hg/wlock` symlink (or file), with lock breaking when the recorded pid and hostname show that a lock was left by a local process that is not running anymore (as it might have been killed). Differential Revision: https://phab.mercurial-scm.org/D11835

rust: Serializing a DirstateMap does not mutate it anymore Differential Revision: https://phab.mercurial-scm.org/D11834

rhg: don't run `blackbox` if not activated You currently have no way of turning off blackbox. Aside from being a bug, this can be annoying if you use `rhg` in your shell prompt, but also because the current implementation of blackbox is quite slow due to `user` querying. Differential Revision: https://phab.mercurial-scm.org/D11813

rust: add function to check if an extension is enabled This will be used in the next patch Differential Revision: https://phab.mercurial-scm.org/D11812

dirstate: remove unused method Its last usage was inlined in `merge.py`. Differential Revision: https://phab.mercurial-scm.org/D11849

python2-compat: fix extensions' tests on python2 They are minor output different to be taken in account. Differential Revision: https://phab.mercurial-scm.org/D11861

rhg: add support for narrow clones and sparse checkouts This adds a minimal support that can be implemented without parsing the narrowspec. We can parse the narrowspec and add support for more operations later. The reason we need so few code changes is as follows: Most operations need no special treatment of sparse because some of them only read dirstate (`rhg files` without `-r`), which bakes in the filtering, some of them only read store (`rhg files -r`, `rhg cat`), and some of them read no data at all (`rhg root`, `rhg debugrequirements`). `status` is the command that might care about sparse, so we just disable rhg on it. For narrow clones, `rhg files` clearly needs the narrowspec to work correctly, so we fall back. `rhg cat` seems to work consistently with `hg cat` if the file exists. If the file is hidden by narrow spec, the error message is different and confusing, so that's something that we should improve in follow-up patches. Differential Revision: https://phab.mercurial-scm.org/D11764

update: reformat the `commands.update.check` help This make the various option clearer. Differential Revision: https://phab.mercurial-scm.org/D11852