[FIX] web: error on change value in o2m after an other field onchange if the o2m contains o2m

o2m editable list open a form view and can put command as default value (record[this.name] for o2m in a o2m) for the editable fields.
When the onchange is triggered, get_value create a wrong command of command.

step to reproduce issue:
* edit a quotation with at least on sale order line
* change the pricelist
* change the quantity of the sale order line

todo : remove _on_load_record in version > 9.0 and use new dataset implementation with to_delete attribute

addons/web/static/src/js/views/form_relational_widgets.js

@@ -466,7 +466,7 @@ var AbstractManyField = common.AbstractField.extend({
         this.starting_ids = [];
         // don't set starting_ids for the new record
         if (record.id && record[this.name] && (!isNaN(record.id) || record.id.indexOf(this.dataset.virtual_id_prefix) === -1)) {
-            this.starting_ids =  record[this.name].slice();
+            this.starting_ids =  this.get('value').slice();
         }
         this.trigger("load_record", record);
     },