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

mercurial/changelog.py

@@ -151,7 +151,7 b' class _changelogrevision(object):'
     manifest = attr.ib(default=nullid)
     user = attr.ib(default='')
     date = attr.ib(default=(0, 0))
-    files = attr.ib(default=[])
+    files = attr.ib(default=attr.Factory(list))
     description = attr.ib(default='')
 
 class changelogrevision(object):