make show stats work when only one file changed (#32244)

fix https://github.com/go-gitea/gitea/issues/32226

in https://github.com/go-gitea/gitea/pull/27775 , it do some changes to
only show diff file tree when more than one file changed. But looks it
also break the `diff-file-list` logic, which looks not expected change.
so try fix it.

/cc @silverwind 

example view:

![image](https://github.com/user-attachments/assets/281e9c4f-a269-4d36-94eb-a132058aea87)

Signed-off-by: a1012112796 <1012112796@qq.com>

web_src/js/features/repo-diff-filetree.ts

@@ -8,7 +8,9 @@ export function initDiffFileTree() {
 
   const fileTreeView = createApp(DiffFileTree);
   fileTreeView.mount(el);
+}
 
+export function initDiffFileList() {
   const fileListElement = document.querySelector('#diff-file-list');
   if (!fileListElement) return;