The compare page hid the diff when a comparison had no commits, even if
the trees differ. A direct comparison whose head is an ancestor of its
base hits this, which is where the Compare link on a force-push comment
leads after commits were dropped.
The diff box now shows whenever there are commits or changed files, with
the header reading "0 Commits".
Fixes: https://github.com/go-gitea/gitea/issues/36390
Co-authored-by: silverwind <me@silverwind.io>