linelog: be more careful about types before looking for _target attribute...
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
