DECIMAL column was used instead of BIGINT for the minimal possible
BIGINT (-9223372036854775808).

The Item_func_neg::fix_length_and_dec has been adjusted to
to inherit the type of the argument in the case when it's an 
Item_int object whose value is equal to LONGLONG_MIN.


sql/item_func.cc:
  Fixed bug #28625.
  The Item_func_neg::fix_length_and_dec has been adjusted to
  to inherit the type of the argument in the case when it's an 
  Item_int object whose value is equal to LONGLONG_MIN.
mysql-test/t/bigint.test:
  Added test result for bug #28625.
mysql-test/r/bigint.result:
  Added test case for bug #28625.