rawdata: implement the method for `unionrepo` too

This is required for all implementations.

mercurial/unionrepo.py

@@ -116,6 +116,9 b' class unionrevlog(revlog.revlog):'
             # already cached
         return text
 
+    def rawdata(self, nodeorrev, _df=None):
+        return self.revision(nodeorrev, _df=_df, raw=True)
+
     def baserevision(self, nodeorrev):
         # Revlog subclasses may override 'revision' method to modify format of
         # content retrieved from revlog. To use unionrevlog with such class one