and merges in subsequent transactions indiscriminately until it's told to stop.
It also gives the last transaction in its child list a chance to merge the
next transaction.
All this is in support of complex transactions that result in text insertion
being able to collapse into a single undoable event.
Also improved tracking of bogus content node used when document is empty.
git-svn-id: svn://10.0.0.236/trunk@24151 18797224-902f-48f8-a5cc-f745e15eee43
