'derived_merge':
  - if the flag is off then all derived tables are materialized
  - if the flag is on then mergeable derived tables are merged
'derived_with_keys':
  - if the flag is off then no keys are created for derived tables
  - if the flag is on then for any derived table a key to access 
    the derived table may be created.

Now by default both flags are on.
Later the default values for the flags will be off to comply with
the current behaviour of mysql-5.1. 

Uncommented previously commented out test case from parts.partition_repair_myisam
after having added an explicit requirement to materialize the derived
table used in the test case.