tables or more

The problem was that the optimizer used the join buffer in cases when
the result set is ordered by filesort. This resulted in the ORDER BY
clause being ignored, and the records being returned in the order
determined by the order of matching records in the last table in join.

Fixed by relaxing the condition in make_join_readinfo() to take
filesort-ordered result sets into account, not only index-ordered ones.


mysql-test/r/select.result:
  Added a test case for bug #30666.
mysql-test/t/select.test:
  Added a test case for bug #30666.
sql/sql_select.cc:
  Relaxed the condition to determine when the join buffer usage must be
  disabled. The condition is now true for cases when the result set is
  ordered by filesort, that is when 'join->order &&
  !join->skip_sort_order' is true.