engine: make hook point for extension a public function
Well there are no private public functions in Python, but we generally treat
functions and variables starting with `_` as private ones.
A function which needs to be overrided in extension should be a public one.
Differential Revision: https://phab.mercurial-scm.org/D9672
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true