linelog: be more careful about types before looking for _target attribute
Without this, pytype (correctly) complains that fakejge might not have
a _cmprev attribute. We're operating on some constraints not
expressible in the type system, so we detect the invalid case and
raise a nicer exception now.
Sadly, we also need an `assert` to give PyType a clue that we know
what we're doing, but I can absolutely live with that.
Differential Revision: https://phab.mercurial-scm.org/D7271
[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