black.toml

rust-status: collect traversed directories if required

Some commands (`hg purge` notably) register the `traversedir` callback on their
matcher to run said callback every time a directory is traversed.

This is the first of three patches, further broadening Rust support for status.

Unfortunately, there is no way around collecting a full `Vec` (or any other
owned datastructure, like a radix tree) and pushing it back up the Python layer
since keeping the Python callback in a closure would mean giving up
multithreading because of the GIL, which is obviously unacceptable.

Performance is still a lot better than the Python+C path.

Running `hg clean/purge` on Netbeans' repo (100k files):

```
     | No-op | 30% unknown
--------------------------
Rust | 1.0s  | 1.67s
C    | 2.0s  | 2.87s
```

Differential Revision: https://phab.mercurial-scm.org/D8518

[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true