lnwallet+breacharbiter: update breach logic to be aware of new commitment format

In this commit, we update the brar logic in the channel state machine,
and also the brar itself to be aware of the new commitment format.
Similar to the unilateral close summary, we'll now blank out the
SingleTweak field in `NewBreachRetribution` if it's a tweakless
commitment. The brar will then use this to properly identify the
commitment type, to ensure we use the proper witness generation function
when we're handling our own breach.

lnwallet/channel.go

@@ -2066,6 +2066,12 @@ func NewBreachRetribution(chanState *channeldb.OpenChannel, stateNum uint64,
 			},
 			HashType: txscript.SigHashAll,
 		}
+
+		// If this is a tweakless commitment, then we can safely blank
+		// out the SingleTweak value as it isn't needed.
+		if tweaklessCommit {
+			localSignDesc.SingleTweak = nil
+		}
 	}
 
 	// Similarly, if the remote balance exceeds the remote party's dust