Refactor diffFileInfo / DiffTreeStore (#24998)

Follow  #21012, #22399

Replace #24983, fix #24938

Help #24956

Now, the `window.config.pageData.diffFileInfo` itself is a reactive
store, so it's quite easy to sync values/states by it, no need to do
"doLoadMoreFiles" or "callback".

Screenshot: these two buttons both work. After complete loading, the UI
is also right.

<details>


![image](cc6310fd-7f27-45ea-ab4f-24952a87b421)


![image](4c11dd67-ac03-4568-8541-91204d27a4e3)


![image](38a22cec-41be-41e6-a209-f347b7a4c1de)

</details>

web_src/js/components/DiffFileTreeItem.vue

@@ -40,7 +40,7 @@
 
 <script>
 import {SvgIcon} from '../svg.js';
-import {DiffTreeStore} from '../modules/stores.js';
+import {diffTreeStore} from '../modules/stores.js';
 
 export default {
   components: {SvgIcon},
@@ -56,7 +56,7 @@ export default {
     },
   },
   data: () => ({
-    store: DiffTreeStore,
+    store: diffTreeStore(),
     collapsed: false,
   }),
   methods: {