rust-hg-path: add method to get part of a path relative to a prefix
This will be used in the next patch in this series.
Differential Revision: https://phab.mercurial-scm.org/D7526
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
| contrib/python-zstandard/
'''
skip-string-normalization = true
quiet = true