Problem #1: INSERT...SELECT
INSERT ... SELECT with the same table on both sides (hidden
below a MERGE table) does now work by buffering the select result.
The duplicate detection works now after open_and_lock_tables() 
on the locks.
I did not find a test case that failed without the change in
sql_update.cc. I made the change anyway as it should in theory
fix a possible MERGE table problem with multi-table update.


mysql-test/r/create.result:
  BUG#5390 - problems with merge tables
  Removed a duplicate test.
mysql-test/r/merge.result:
  BUG#5390 - problems with merge tables
  Problem #1: INSERT...SELECT
  Added test results.
mysql-test/t/create.test:
  BUG#5390 - problems with merge tables
  Removed a duplicate test.
mysql-test/t/merge.test:
  BUG#5390 - problems with merge tables
  Problem #1: INSERT...SELECT
  Added tests.
sql/lock.cc:
  BUG#5390 - problems with merge tables
  Problem #1: INSERT...SELECT
  Added a new function to find a duplicate lock in a list of tables.
sql/mysql_priv.h:
  BUG#5390 - problems with merge tables
  Problem #1: INSERT...SELECT
  Added a declaration for the new function.
sql/sql_parse.cc:
  BUG#5390 - problems with merge tables
  Problem #1: INSERT...SELECT
  Changed the duplicate tables detection for INSERT ... SELECT
  to use the new function, which does also work for MERGE tables.
sql/sql_update.cc:
  BUG#5390 - problems with merge tables
  Changed the duplicate tables detection for UPDATE
  to use the new function, which does also work for MERGE tables.