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:

Signed-off-by: a1012112796 <1012112796@qq.com>
(cherry picked from commit c4b2808b896dd86323c6a0d119c8cf24752d4d8a)
Signed-off-by: Gergely Nagy <forgejo@gergo.csillger.hu>
Conflicts:
- web_src/js/features/repo-diff-filetree.js
web_src/js/features/repo-diff.js
Conflicts resolved by manually applying the changes.