- In QUICK_INDEX_MERGE_SELECT::read_keys_and_merge: when we got table->sort from Unique,
  tell init_read_record() not to use rr_from_cache() because a) rowids are already sorted
  and b) it might be that the the data is used by filesort(), which will need record rowids
  (which rr_from_cache() cannot provide).
- Fully de-initialize the table->sort read in QUICK_INDEX_MERGE_SELECT::get_next(). This fixes BUG#35477.
(bk trigger: file as fix for BUG#35478).

sql/filesort.cc:
  BUG#35478: sort_union() returns bad data when sort_buffer_size is hit
  - make find_all_keys() use quick->get_next() instead of init_read_record(r)/r.read_record() calls
  - added dbug printout
sql/mysql_priv.h:
  BUG#35478: sort_union() returns bad data when sort_buffer_size is hit
  - Added parameter to init_read_record
sql/opt_range.cc:
  BUG#35478: sort_union() returns bad data when sort_buffer_size is hit
  - In QUICK_INDEX_MERGE_SELECT::read_keys_and_merge: when we got table->sort from Unique,
    tell init_read_record() not to use rr_from_cache() because a) rowids are already sorted
    and b) it might be that the the data is used by filesort(), which will need record rowids
    (which rr_from_cache() cannot provide).
  - Fully de-initialize the table->sort read in QUICK_INDEX_MERGE_SELECT::get_next().
sql/records.cc:
  BUG#35478: sort_union() returns bad data when sort_buffer_size is hit
  - Added disable_rr_cache parameter to init_read_record
  - Added comment
sql/sql_acl.cc:
  BUG#35478: sort_union() returns bad data when sort_buffer_size is hit
  - Added parameter to init_read_record
sql/sql_delete.cc:
  BUG#35478: sort_union() returns bad data when sort_buffer_size is hit
  - Added parameter to init_read_record
sql/sql_help.cc:
  BUG#35478: sort_union() returns bad data when sort_buffer_size is hit
  - Added parameter to init_read_record
sql/sql_select.cc:
  BUG#35478: sort_union() returns bad data when sort_buffer_size is hit
  - Added parameter to init_read_record
sql/sql_table.cc:
  BUG#35478: sort_union() returns bad data when sort_buffer_size is hit
  - Added parameter to init_read_record
sql/sql_udf.cc:
  BUG#35478: sort_union() returns bad data when sort_buffer_size is hit
  - Added parameter to init_read_record
sql/sql_update.cc:
  BUG#35478: sort_union() returns bad data when sort_buffer_size is hit
  - Added parameter to init_read_record