This will help us in having an API where we can pass the filtername and the
visibilityexceptions to get a new repo object.
Visibility exceptions are the revs which must be visible even they should in
theory belong to the hidden set. They are required as there has been desire to
have a functionality to access hidden changesets using certain commands without
passing --hidden. After this patch we can make those changesets visibility
exceptions so that we can access them without requiring a unfiltered repo.
We have to use object.__setattr__() because __setattr__() is
proxied to the unfilteredrepo.