When recalculating parent ways/relations during rebase, a
graph should not add modified or deleted entities as parents.
Such entities will already be correctly marked as parents or
not.
Graph had the correct behavior for deleted entities, but not for
modified entities. This had the effect that if you split a way
that was partially off screen, and then panned so that the way was
re-retrieved, Graph#rebase would mistakenly add back the original
way as a parent of all the nodes that were split into the new
section, making them appear as shared.
Fixes#751.
