commitctx: extract _filecommit too
This function is exclusively used in `commitctx`. So we should extract it too
for consistency and to reduce the `localrepo` bloat.
This is part of a larger refactoring/cleanup of the commitctx code to clarify
and augment the logic gathering metadata useful for copy tracing. The current
code is a tad too long and entangled to make such update easy.
Differential Revision: https://phab.mercurial-scm.org/D8710
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true