The logic for computing reference traces was unintentionally finding the
*longest* possible trace (approximately). I think I already tried to fix
this before, but misunderstood how my own code works. Here, we fix it
properly: by slightly reworking the logic to use one ArrayHashMap for
both the result and the traversal queue, we trivially get a proper
breadth-first traversal so that we can find the shortest possible
reference trace for every referenced unit.
8907dc8a4f