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