Added test cases for bug #12863.
item_sum.cc, item_sum.h:
  Fixed bug #12863.
  Added a flag to Item_func_group_concat set to FALSE after
  concatenation of the first element of a group.


sql/item_sum.h:
  Fixed bug #12863.
  Added a flag to Item_func_group_concat set to FALSE after
  concatenation of the first element of a group.
sql/item_sum.cc:
  Fixed bug #12863.
  Added a flag to Item_func_group_concat set to FALSE after
  concatenation of the first element of a group.
mysql-test/t/func_gconcat.test:
  Added test cases for bug #12863.
mysql-test/r/func_gconcat.result:
  Added test cases for bug #12863.