ensure hasSHAChanged == true when setData includes a different SHA from constructor

spec/javascripts/vue_mr_widget/stores/mr_widget_store_spec.js

+import MergeRequestStore from '~/vue_merge_request_widget/stores/mr_widget_store';
+import mockData from '../mock_data';
+
+describe('MergeRequestStore', () => {
+  describe('setData', () => {
+    let store;
+
+    beforeEach(() => {
+      store = new MergeRequestStore(mockData);
+    });
+
+    it('should set hasSHAChanged when the diff SHA changes', () => {
+      store.setData({ ...mockData, diff_head_sha: 'a-different-string' });
+      expect(store.hasSHAChanged).toBe(true);
+    });
+
+    it('should not set hasSHAChanged when other data changes', () => {
+      store.setData({ ...mockData, work_in_progress: !mockData.work_in_progress });
+      expect(store.hasSHAChanged).toBe(false);
+    });
+  });
+});