If the document is edited while redlines are shown, the node merge flags
aren't updated because only CheckParaRedlineMerge() really updates the
flags.

But CheckParaRedlineMerge() knows what the last node of a merge is,
so we can conclude that the next node then isn't merged and needs to
have frames created for it.

Change-Id: Ie9e189a2d9251910c3f5ac98a46867bbe1d91c10