revlog: move index reading logic in a dedicated method
They are multiple motivation to do it:
* The logic is complicated enough to deserver its own method.
* We will need to reuse this once we put a docket in use.
* This split the actual reading from the processing of the read data better.
Differential Revision: https://phab.mercurial-scm.org/D10595
[build-system]
requires = ["setuptools", "wheel"]
build-backend = "setuptools.build_meta"
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true