rebase: use cmdutil.check_at_most_one_arg() for action
Here we also needed to know what the action was (if any), so I've
updated the helper to return any specified option.
Differential Revision: https://phab.mercurial-scm.org/D7640
[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