[FIX] web: do not change pager when discarding changes

Here is a scenario that could happen before this commit:
1. go to a list view with more than one record, say 3
2. click on the 3rd record to open the form view
3. the pager says 3/3
4. click on edit button
5. click on discard button
6. the pager says 1/3 (but the record displayed is still the same)

It is often necessary to restore the offset for the sub records, because
the number of pages in a one2many could have changed.  However, it is
not a good idea to do that for the main record, since it interferes with
the usual flow of operations.

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

@@ -347,6 +347,7 @@ var BasicModel = AbstractModel.extend({
         var element = this.localData[id];
         var isNew = this.isNew(id);
         var rollback = 'rollback' in options ? options.rollback : isNew;
+        var initialOffset = element.offset;
         this._visitChildren(element, function (elem) {
             if (rollback && elem._savePoint) {
                 if (elem._savePoint instanceof Array) {
@@ -365,6 +366,7 @@ var BasicModel = AbstractModel.extend({
                 delete elem.tempLimitIncrement;
             }
         });
+        element.offset = initialOffset;
     },
     /**
      * Duplicate a record (by calling the 'copy' route)

addons/web/static/tests/views/form_tests.js

@@ -6525,5 +6525,35 @@ QUnit.module('Views', {
         testUtils.unpatch(mixins.ParentedMixin);
     });
 
+    QUnit.test('do not change pager when discarding current record', function (assert) {
+        assert.expect(2);
+
+        var form = createView({
+            View: FormView,
+            model: 'partner',
+            data: this.data,
+            arch: '<form string="Partners">' +
+                    '<field name="foo"/>' +
+                '</form>',
+            viewOptions: {
+                ids: [1, 2],
+                index: 0,
+            },
+            res_id: 2,
+        });
+
+        assert.strictEqual(form.pager.$('.o_pager_counter').text().trim(), '2 / 2',
+            'pager should indicate that we are on second record');
+
+        form.$buttons.find('.o_form_button_edit').click();
+        form.$buttons.find('.o_form_button_cancel').click();
+
+        assert.strictEqual(form.pager.$('.o_pager_counter').text().trim(), '2 / 2',
+            'pager should not have changed');
+
+        form.destroy();
+    });
+
+
 });
 });