- return HA_KEY_SCAN_NOT_ROR flag for HASH indexes;
- Fix ha_heap::cmp_ref() to work with BTREE index scans.


mysql-test/r/index_merge.result:
  BUG#29740: testcase
mysql-test/t/index_merge.test:
  BUG#29740: testcase
sql/ha_heap.h:
  BUG#29740: Wrong query results for index_merge/union over HEAP table.
  - make HEAP table engine return HA_KEY_SCAN_NOT_ROR flag for HASH 
    indexes,as HASH index does not guarantee any ordering for rows
    within the hash bucket.
  - Fix BTREE indexes: make ha_heap::cmp_ref() compare the rowids in the 
    same way as ha_key_cmp() does.
sql/opt_range.cc:
  BUG#29740: Fix comment about ROR scans.