.arcconfig

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

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