Martin von Zweigbergk <martinvonz@google.com> [Wed, 11 Mar 2015 21:36:48 -0700] rev 24287
largefiles: don't create chain of __contains__ calls
Matt Harbison pointed out that my recent 2720f967a7b1 might cause
__contains__ to continously get replaced by another version that calls
itself, since the manifest instance returned by the super method is
always the same instance due to @propertycache. He also suggested
replacing the class instead, as is done with the context class in the
surrounding code. Do so.