changelog: use a Factory for default value for files The default value is compiled into the generated type. This means that default values are shared between instances. For immutable types like bool, str, int, and tuple, this is fine. But for mutable types like list and dict, we need to use attr.Factory() to instantiate a new instance of the default for each object. Differential Revision: https://phab.mercurial-scm.org/D901