repoview: prevent `None` to be passed as the filtername

We let such instantiation slip in a previous commit, so we add an explicit check
to prevent it to happen in the future.

mercurial/repoview.py

@@ -397,6 +397,9 b' class repoview:'
     """
 
     def __init__(self, repo, filtername, visibilityexceptions=None):
+        if filtername is None:
+            msg = "repoview should have a non-None filtername"
+            raise error.ProgrammingError(msg)
         object.__setattr__(self, '_unfilteredrepo', repo)
         object.__setattr__(self, 'filtername', filtername)
         object.__setattr__(self, '_clcachekey', None)