[FIX] web: prevent crash in some cases (o2m)

Recent works in the orm (see [1] for example) changed the behaviour of
the server in the case of onchanges: it tries to optimize Odoo by
sending a minimal diff to the web client in onchanges.

Concretely, this means that the web client receives commands 4 (link to)
instead of command 1 (update + record data). It also sends a minimal
diff in that case. This is a big issue for the web client, because it relied
on the knowledge of the data from the command 1 to be able to evaluate some
modifiers.

The good news is that we can mitigate the issue in one case: we do not
actually need to evaluate the modifiers in the case of a command 4
(comming from the server), because we can assume that the record is
valid. So, we simply bypass the check in that case.

Note that this should fix most of the problem, but a deeper problem
still remains: a crash can still happen when an onchange modifies a
record in a one2many (but in a different page), and we also have a
modifier which needs to be evaluated.

opw: #1904514

[1] https://github.com/odoo/odoo/pull/29086

closes odoo/odoo#29172

closes odoo/odoo#29086

addons/web/static/src/js/views/basic/basic_model.js

@@ -3208,6 +3208,7 @@ var BasicModel = AbstractModel.extend({
         _.each(element._changes, function (command) {
             if (command.operation === 'DELETE' ||
                     command.operation === 'FORGET' ||
+                    (command.operation === 'ADD' &&  !command.isNew)||
                     command.operation === 'REMOVE_ALL') {
                 return;
             }

addons/web/static/tests/fields/relational_fields_tests.js

@@ -6207,6 +6207,51 @@ QUnit.module('relational_fields', {
         form.destroy();
     });
 
+    QUnit.test('one2many, onchange, edition and multipage...', function (assert) {
+        assert.expect(7);
+
+        this.data.partner.onchanges = {
+            turtles: function (obj) {
+                obj.turtles = [[5]].concat(obj.turtles);
+            }
+        };
+
+        this.data.partner.records[0].turtles = [1,2,3];
+
+        var form = createView({
+            View: FormView,
+            model: 'partner',
+            data: this.data,
+            arch:'<form string="Partners">' +
+                    '<field name="turtles">' +
+                        '<tree editable="bottom" limit="2">' +
+                            '<field name="turtle_foo"/>' +
+                        '</tree>' +
+                    '</field>' +
+                '</form>',
+            res_id: 1,
+            mockRPC: function (route, args) {
+                assert.step(args.method + ' ' + args.model)
+                return this._super(route, args);
+            },
+            viewOptions: {
+                mode: 'edit',
+            },
+        });
+        form.$('.o_field_x2many_list_row_add a').click();
+        form.$('.o_field_x2many_list_row_add a').click();
+
+        assert.verifySteps([
+            'read partner',
+            'read turtle',
+            'default_get turtle',
+            'onchange partner',
+            'default_get turtle',
+            'onchange partner',
+        ]);
+        form.destroy();
+    });
+
     QUnit.test('one2many with CREATE onchanges correctly refreshed', function (assert) {
         assert.expect(5);