rust-chg: add helper to pack environment variables
On my machine, "printenv | wc -c" says 3422. That's the only reason why
the initial buffer capacity is set to 4kB.
Differential Revision: https://phab.mercurial-scm.org/D8363
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true