lnwallet: nil single tweak when creating keyring

To make the channel state machine less concerned about the type of
commitment, we nil the local tweak when creating the keyring, depending
on the commitment type.

lnwallet/commitment.go

@@ -146,6 +146,13 @@ func DeriveCommitmentKeys(commitPoint *btcec.PublicKey,
 	// we'll use that directly, and ignore the commitPoint tweak.
 	if tweaklessCommit {
 		keyRing.ToRemoteKey = toRemoteBasePoint
+
+		// If this is not our commitment, the above ToRemoteKey will be
+		// ours, and we blank out the local commitment tweak to
+		// indicate that the key should not be tweaked when signing.
+		if !isOurCommit {
+			keyRing.LocalCommitKeyTweak = nil
+		}
 	} else {
 		keyRing.ToRemoteKey = input.TweakPubKey(
 			toRemoteBasePoint, commitPoint,