sql/sql_class.h:
  small optimisations for MULTI-TABLE UPDATE's
sql/sql_parse.cc:
  Could not look at that if () at the start of execute_command, and as 
  Monty evidently did not have time, so I optimised it away in the best 
  fashion I know of.
sql/sql_update.cc:
  Some optimisations  for MULTI-TABLE UPDATE's