rhg: allow setting defaults.cmd to an empty string
This is used by at least one hg UI to prevent defaults from affecting
the command:
https://github.com/emacs-mirror/emacs/blob/b71beb7ae7c60a5c5af608420d28fdda5265a264/lisp/vc/vc-hg.el#L245
Let's let it work.
[build-system]
requires = ["setuptools", "wheel"]
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true