copies-rust: extract the processing of a single copy information This will make it easy to process copy from both p1 and p2 in the same `add_from_changes` call. Differential Revision: https://phab.mercurial-scm.org/D9648

[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true