committablectx: override manifestnode() to return None

wctx.manifestnode() crashed before because it has no _changeset. Instead of
crashing, just return None like wctx.node().

mercurial/context.py

@@ -1191,6 +1191,8 b' class committablectx(basectx):'
     def subrev(self, subpath):
         return None
 
+    def manifestnode(self):
+        return None
     def user(self):
         return self._user or self._repo.ui.username()
     def date(self):