Merged undos don't decrement dirtyCounter correctly

[Chonne]

This is following the implementation of undo merging.

When the mergeUndoDeltas option is set to true, if a user types a few characters at once then executes the undo() command, the UndoManager's dirtyCounter variable is only decremented once instead of being decremented by the number of merged undos. Note that this will probably happen for any merged undo commands.

This makes it difficult to check if a document is clean or, rendering the UndoManager's isClean() method useless.