Bug in parent computation in the manager

[breuleux]

This issue pops up in commit 740731c for test test_infer::test_raise_rec. It is not deterministic, so you may have to run the test multiple times. The error is Too many parents for graph xf

The error occurs early, from cloning the freshly parsed graph.

Changing L465 to reach = OrderedSet() solves the issue, which suggests that some orderings of reach are problematic.