rawdata: register the method for `ifiledata`

The interface have a `revision(..., raw=False)` method so it should get a
`rawdata` one. I am not sure why nothing complained about the lack of it
earlier.

mercurial/repository.py

@@ -597,6 +597,10 b' class ifiledata(interfaceutil.Interface)'
         consumers should use ``read()`` to obtain the actual file data.
         """
 
+    def rawdata(node):
+        """Obtain raw data for a node.
+        """
+
     def read(node):
         """Resolve file fulltext data.