merge_bbs creating too many copies of self-loop edge

[acl-cqc]

This relates to #1144 which reveals the bug (by making the Hugr fail validation), previously the multiple-edges had been going unnoticed.

The merge_bbs.rs test in_loop with self_loop: true replaces two BBs with one BB that has an edge back to itself. After performing the Replacement, the new BB in the Hugr has four edges back to itself (all from the same port).
I've checked the Replacement - the replacement Hugr contains exactly one self-edge, mu_out contains exactly one edge leaving the replacement, mu_inp is empty - all as expected.

The problem appears to arise immediately after this:

hugr/hugr-core/src/hugr/rewrite/replace.rs

Line 282 in 1032b7a

let InsertionResult { new_root, node_map } = h.insert_hugr(parent, self.replacement);

i.e. before we even inspect mu_inp / mu_out etc. So I believe this is due to CQCL/portgraph#130 but I'm not clear why we see four copies of the edge as from that portgraph issue I would expect to see two.

[aborgna-q]

The next portgraph release should fix that. See CQCL/portgraph#132

[aborgna-q]

This is now published in portgraph 0.12.1, so we should update it here.