Probe up to second-to-last hop if last was provided by route hint

If the last hop was provided by route hint we assume it's not an announced channel.
If furthermore only a single route hint is provided we refrain from probing through
all the way to the end and instead probe up to the second-to-last channel.

Optimally we'd do this not based on above mentioned assumption but
rather by checking inclusion in our network graph. However, we don't
have access to our graph in `ChannelManager`.

Author: tnull

lightning/src/ln/channelmanager.rs

@@ -3520,7 +3520,26 @@ where
 		let mut used_liquidity_map = HashMap::with_capacity(first_hops.len());
 
 		let mut res = Vec::new();
-		for path in route.paths {
+		for mut path in route.paths {
+			// If the last hop was provided by route hint we assume it's not an announced channel.
+			// If furthermore only a single route hint is provided we refrain from probing through
+			// all the way to the end and instead probe up to the second-to-last channel.
+			if let Payee::Clear { node_id, ref route_hints, .. } = route_params.payment_params.payee {
+				if route_hints.len() == 1 {
+					if let Some(hint) = route_hints[0].0.iter().last() {
+						if let Some(last_path_hop) = path.hops.last() {
+							if last_path_hop.pubkey == node_id && last_path_hop.short_channel_id == hint.short_channel_id {
+								let final_value_msat = path.final_value_msat();
+								path.hops.pop();
+								if let Some(mut new_last) = path.hops.iter_mut().last() {
+									new_last.fee_msat += final_value_msat;
+								}
+							}
+						}
+					}
+				}
+			}
+
 			if path.hops.len() + path.blinded_tail.as_ref().map_or(0, |t| t.hops.len()) < 2 {
 				log_debug!(
 					self.logger,