This finally fixes in the bug addressed by commit 309ddbbd, where
parallel edges could be erroneously created. In fact, the bug was more
serious: The source and target edge lists always pointed to the same
list (in the uncorrelated case, but could occasionally happen for the
correlated case also) which got shuffled during iteration. Since the
shuffling of one list interfered with the shuffling of the other, some
combinations of source and target edges could simply never be
considered... This commit forces both lists to always be independent.