mysql-test/r/func_gconcat.result:
  Test result fixed.
sql/item_sum.cc:
  Cleanup of Item_func_group_concat:
  - last unobvious things commented
  - don't store NULLs in the tree.
  - remove unneeded variables.
  - use setup_order, not setup_group to setup group concat order list
sql/item_sum.h:
  Cleanup for Item_func_group_concat: remove unneded variables, methods,
  move a bunch of variables to private: section.