Fix for BUG#7377. This fix adds the same implementation for ha_myisammgr::index_type as in version 5.0.


mysql-test/r/merge.result:
  Added test result for BUG#7377.
mysql-test/t/merge.test:
  Added test for BUG#7377.
sql/ha_myisammrg.cc:
  Added implementation for handler::index_type.
sql/ha_myisammrg.h:
  Added implementation for handler::index_type.