debugdeltachain: detect a special case where parents are "skipped"

See inline comment for details, this is a case where the delta is neither
against p1 or p2, Yet it is still a simple delta part of a simple chain.

We now display them as `skip1/skip2` instead of `other`.
12 jobs for topic/default/better-debug-delta-chain in 46 minutes and 52 seconds (queued for 4 seconds)