drawdag: provide filenode for its dummy filectx

basefctx has filenode, which will be used in a future fast path of skipping
flag processor during repo._filecommit.

This patch adds filenode to drawdag's faked filectx so we don't need to use
hasattr in localrepo.py.

tests/drawdag.py

@@ -214,6 +214,9 b' class simplefilectx(object):'
     def data(self):
         return self._data
 
+    def filenode(self):
+        return None
+
     def path(self):
         return self._path