rawdata: implement `rawdata` for `simplestore` too

This is needed for all implementation.

tests/simplestorerepo.py

@@ -326,6 +326,9 b' class filestorage(object):'
 
         return text
 
+    def rawdata(self, nodeorrev):
+        return self.revision(raw=True)
+
     def read(self, node):
         validatenode(node)