copies: return None instead of ChangingFiles when relevant
If they are not relevant data, they are no need to fetch them.
Differential Revision: https://phab.mercurial-scm.org/D9140
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true