When setting AUTOCOMMIT=1 after starting a transaction, the binary log
did not commit the outstanding transaction. The reason was that the binary
log commit function saw the values of the new settings, deciding that there
were nothing to commit.

Fixed the problem by moving the implicit commit to before the thread option
flags were changed, so that the binary log sees the old values of the flags
instead of the values they will take after the statement.


mysql-test/extra/binlog_tests/implicit.test:
  New test file to check implicit commits both inside and outside transactions.
mysql-test/suite/binlog/t/binlog_implicit_commit.test:
  Test for implicit commit of SET AUTOCOMMIT and LOCK/UNLOCK TABLES.
sql/set_var.cc:
  Adding code to commit pending transaction before changing option flags.