copies: avoid materializing a full directory map during copy tracing
Materializing a full copy of every directory in a treemanifest repo can be quite
expensive, even with a narrow matcher. For flat manifest repos, this should be
equivalent - it will still materialize (and cache) a dict of all of the dirs
inside of the manifest object, we just don't get a copy of it.
In a repo I have here, this brings the time for a simple rebase from 11.197s to
4.609s.
Differential Revision: https://phab.mercurial-scm.org/D9503
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true