py3: return False early while checking whether None is a key in lazymanifest

In this patch we returns False early if we are trying to check whether None is a
key in lazymanifest. The reason I added a diff is that on Python 3, it goes into
the lazy manifest code an returns a TypeError. I checked with Durham that
whether None can be a possible key in lazymanifest and he said "no". So it's
safe to have this if statement.

This fixes `hg merge` on Python 3.

Differential Revision: https://phab.mercurial-scm.org/D852

mercurial/manifest.py

@@ -442,6 +442,8 b' class manifestdict(object):'
         self._lm[key] = node, self.flags(key, '')
 
     def __contains__(self, key):
+        if key is None:
+            return False
         return key in self._lm
 
     def __delitem__(self, key):