graphmod: fix seen state handling for > 2 parents (issue5174) When there are more than 2 parents for a given node (in a sparse graph), extra dummy nodes are inserted to transition the lines more gradually. However, since the seen state was not updated when yielding the extra nodes, the wrong graph styles were being applied to the nodes.

ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.