fix: sync: do not include incoming in return of syncFork

[arajasek]

Related Issues

Both @TippyFlitsUK and I ran into failures on this line when running latest master. This is due to a bug in #11533, which causes us to return one EXTRA tipset in the return. This return then gets folded into the blockSet, which includes the incoming tipset already, leading to us having a non-continuous "chain".

Proposed Changes

Only return incomingParents in this case.

Additional Info

Please don't merge until I've done some additional testing.

Checklist

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