Found a bug in OwnershipGraph.java when writing out the .dot file. If

Found a bug in OwnershipGraph.java when writing out the .dot file.  If
a heap region that is listed as a heap root, such as an incoming parameter,
and some other heap region creates a reference to it, then it will be
traversed twice, once from it's own heap root starting point, and once when
the referencing heap region's heap root is traversed.  Altered the graph
writing code to use a shared hashset to decide if a heap region has been
visited.