copies: replace duplicatecopies() by function that takes contexts
The callers mostly have context objects, so let's avoid looking up the
same context objects inside `duplicatecopies()`.
I also renamed the function to `graftcopies()` since I think that
better matches its purpose. I did it in the same commit so it's easier
for extensions to switch between the functions.
Differential Revision: https://phab.mercurial-scm.org/D7858
[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