[IMP] point_of_sale: better merge detection

If sells
1. product A
2. product B
3. product A

the lines of product A were not merged

Closes #25540

closes odoo/odoo#26602

addons/point_of_sale/static/src/js/models.js

@@ -2343,10 +2343,11 @@ exports.Order = Backbone.Model.extend({
         }
         if (to_merge_orderline){
             to_merge_orderline.merge(line);
+            this.select_orderline(to_merge_orderline);
         } else {
             this.orderlines.add(line);
+            this.select_orderline(this.get_last_orderline());
         }
-        this.select_orderline(this.get_last_orderline());
 
         if(line.has_product_lot){
             this.display_lot_popup();