Improvements to fostered transclusion wrapping

 * If a transclusion that starts in a table produced fostered content
   and siblings, we were inserting the wrapping metas in between,
   resulting in interweaved markers.

 * A complication is that sometimes newlines show up in between so we
   take care to backtrack when we don't find any more siblings.

Bug: 63790

Change-Id: I732167697a59465cbee4e14a0d493883556c0833