not equivalent to "x=y" when item->negated == TRUE.


mysql-test/r/func_in.result:
  Testcase for BUG#13419
mysql-test/t/func_in.test:
  Testcase for BUG#13419
sql/sql_select.cc:
  Fix for BUG#13419:
  * Take into account that item=Item_func_in(x,y) is not equivalent to "x=y" when 
    item->negated == TRUE.
  * Removed comment that is no longer true.