lnwallet: fix bug in deriveMusig2Shachain

In this commit, we fix a bug in the `deriveMusig2Shachain` function
where it didn't actually use the passed in revocation root as part of
the hmac invocation.

We also modify the function to be more generally useable as well, as now
the caller can just pass in the revocation root things should be derived
from.

lnwallet/channel.go

@@ -1396,17 +1396,9 @@ func NewLightningChannel(signer input.Signer,
 
 	logPrefix := fmt.Sprintf("ChannelPoint(%v):", state.FundingOutpoint)
 
-	// In order to obtain the revocation root hash to create the taproot
-	// revocation, we'll encode the producer into a buffer, then use that
-	// to derive the shachain root needed.
-	var rootHashBuf bytes.Buffer
-	if err := state.RevocationProducer.Encode(&rootHashBuf); err != nil {
-		return nil, fmt.Errorf("unable to encode producer: %v", err)
-	}
-
-	revRootHash := chainhash.HashH(rootHashBuf.Bytes())
-
-	taprootNonceProducer, err := deriveMusig2Shachain(revRootHash)
+	taprootNonceProducer, err := deriveMusig2Shachain(
+		state.RevocationProducer,
+	)
 	if err != nil {
 		return nil, fmt.Errorf("unable to derive shachain: %v", err)
 	}