changectx: add a "maybe filtered" filtered attribute
There are changeset that we know not to be filtered (eg: `null`). In this case,
we could access some information without triggering changelog filtering. We add
an attribute to changectx that track this property.
Differential Revision: https://phab.mercurial-scm.org/D7483
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
| contrib/python-zstandard/
'''
skip-string-normalization = true
quiet = true