"index_merge: fair choice between index_merge union and range access"

mysql-test/include/world.inc:
  A new include file to upload the world database.
mysql-test/include/world_schema.inc:
  A new include file to create tables of the world database.
mysql-test/r/index_merge_myisam.result:
  The results for test cases testing the optimizations added in WL#24 for MyISAM.
mysql-test/r/range_vs_index_merge.result:
  The results for test cases testing the optimizations added in WL#24 for InnoDB.
mysql-test/t/range_vs_index_merge.test:
  Test cases to test the optimizations added in WL#24 for MyISAM.
mysql-test/t/range_vs_index_merge_innodb.test:
  Test cases to test the optimizations added in WL#24 for InnoDB.
sql/sql_list.h:
  Fixed a bug that did not allow adding a non-empty list to an empty list.