listpeers: add 'scratch_txid' for the tx we would broadcast if necessary.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

CHANGELOG.md

@@ -8,6 +8,8 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 ### Added
 
+- JSON API: `listpeers` has new field `scratch_txid`: the latest tx in channel.
+
 ### Changed
 
 ### Deprecated

lightningd/peer_control.c

@@ -642,6 +642,12 @@ static void json_add_peer(struct lightningd *ld,
 		json_object_start(response, NULL);
 		json_add_string(response, "state",
 				channel_state_name(channel));
+		if (channel->last_tx) {
+			struct bitcoin_txid txid;
+			bitcoin_txid(channel->last_tx, &txid);
+
+			json_add_txid(response, "scratch_txid", &txid);
+		}
 		if (channel->owner)
 			json_add_string(response, "owner",
 					channel->owner->name);