manifest: disallow setting the node id of an entry to None

manifest.diff() uses None as a special value to denote the absence of
a file, so setting a file node to None means you then can't trust
manifest.diff().

This should also make future manifest work slightly easier.

mercurial/manifest.py

@@ -17,6 +17,9 b' class manifestdict(dict):'
             flags = {}
         dict.__init__(self, mapping)
         self._flags = flags
+    def __setitem__(self, k, v):
+        assert v is not None
+        dict.__setitem__(self, k, v)
     def flags(self, f):
         return self._flags.get(f, "")
     def withflags(self):