Mutator returns inclusion proof in wrong epoch

[liamsi]

The mutator should return inclusion proofs (for mutations) in the previous epoch. Currently, it returns inclusion proofs in the requested epoch:
The relevant parts are these lines in mutation.go:

keytransparency/core/mutation/mutation.go

Lines 109 to 120 in 5a30e36

	proofs, err := s.inclusionProofs(ctx, indexes, in.Epoch)
	if err != nil {
	return nil, err
	}
	for i, p := range proofs {
	mutations[i].Proof = p
	}
	// MapRevisions start at 1. Log leave's index starts at 0.
	// MapRevision should be at least 1 since the Signer is
	// supposed to create at least one revision on startup.
	respEpoch := resp.GetMapRoot().GetMapRevision() - 1

(should be s.inclusionProofs(ctx, indexes, in.Epoch-1)