- make_join_readinfo() has the code that forces use of "Using temporary; 
  Using filesort" when join buffering is in use.
  That code didn't handle all cases, in particular it didn't hande the case 
  where ORDER BY originally has tables from multiple columns, but the 
  optimizer eventually figures out that doing filesort() on one table 
  will be sufficient.  Adjusted the code to handle that case.