rust: add comment about lack of wdirrev handling

If hg is compiled with rust support, 'only(wdir())' crashed as
"rustext.GraphError: ('ParentOutOfRange', 2147483647)", which should instead
say "abort: working directory revision cannot be specified."

mercurial/revlog.py

@@ -896,6 +896,8 b' class revlog(object):'
             common = [nullrev]
 
         if rustext is not None:
+            # TODO: WdirUnsupported should be raised instead of GraphError
+            # if common includes wdirrev
             return rustext.ancestor.MissingAncestors(self.index, common)
         return ancestor.incrementalmissingancestors(self.parentrevs, common)