When merging a child graph into it's parents graph we
can specialize checking for overlaps to avoid some of
overhead of the default subgraph algorithm (which builds
a full graph, with edges) since we only care about the
number of duplicate nodes (not needing a full subgraph
with retained edges).
Change-Id: Ib211460c58efca3ddb5a254da11aafe44716a639
fd4772ca84