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.

hgext/largefiles/reposetup.py

@@ -44,11 +44,12 b' def reposetup(ui, repo):'
                         return [lfutil.splitstandin(f) or f for f in filenames]
                     def manifest(self):
                         man1 = super(lfilesctx, self).manifest()
-                        orig = man1.__contains__
+                        class lfilesmanifest(man1.__class):
                         def __contains__(self, filename):
+                                orig = super(lfilesmanifest, self).__contains__
                             return (orig(filename) or
                                     orig(lfutil.standin(filename)))
-                        man1.__contains__ = __contains__.__get__(man1)
+                        man1.__class__ = lfilesmanifest
                         return man1
                     def filectx(self, path, fileid=None, filelog=None):
                         orig = super(lfilesctx, self).filectx