Simplest possible fix for CORE-4618: Rollback doesn`t undo changes when MERGE statement updates the same target rows multiple times and PLAN MERGE is used.

git-svn-id: svn+ssh://svn.code.sf.net/p/firebird/code/firebird/branches/B2_5_Release@60312 65644016-39b1-43b1-bf79-96bc8fe82c15