revlog: refactor logic to compute nodemap file in separate function
I will like to use it one more place.
Differential Revision: https://phab.mercurial-scm.org/D9933
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true