rust-dirs-multiset: add `DirsChildrenMultiset`
In a future patch, this structure will be needed to store information needed by
the (also upcoming) `IgnoreMatcher`.
Differential Revision: https://phab.mercurial-scm.org/D7869
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true