Handle unmatched fragment types in diffRelayQuery

[josephsavona]

RelayQueryWriter no longer writes the results of unmatched fragments. However, diffRelayQuery still attempts to diff fragments that don't match the known type. The logic is:

• when diffing, we shouldn't look at fragments whose types don't match
• but we can't remove those fragments, because if we end up refetching the node, it might be a different record whose type could match that fragment
• so we should treat non-matching fragments as requisite fields and only keep them if the node would otherwise be refetched