cext: fix compiler warning about sign changing
line.len is a Py_ssize_t, and we're casing to size_t (unsigned). On my compiler,
this causes a warning to be emitted:
```
mercurial/cext/manifest.c: In function 'pathlen':
mercurial/cext/manifest.c:48:44: warning: operand of ?: changes signedness from 'Py_ssize_t' {aka 'long int'} to 'long unsigned int' due to unsignedness of other operand [-Wsign-compare]
return (end) ? (size_t)(end - l->start) : l->len;
^~~~~~
```
Differential Revision: https://phab.mercurial-scm.org/D7913
[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