depending on table order

multi_update::send_data() was counting updates, not updated rows. Thus if one 
record have several updates it will be counted several times in 'rows matched'
but updated only once.

multi_update::send_data() now counts only unique rows.


sql/sql_update.cc:
  Fix bug#15028  Multitable update returns different numbers of matched rows
  depending on table order
  multi_update::send_data() now counts only unique rows.
mysql-test/t/update.test:
  Test case for bug#15028 Multitable update returns different numbers of matched rows depending on table order
mysql-test/r/update.result:
  Test case for bug#15028 Multitable update returns different numbers of matched rows depending on table order