support the open-icon of folder (#34168) (#34896)

Backport #34168 by @kerwin612

Co-authored-by: Kerwin Bryant <kerwin612@qq.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/repo/diff/box.tmpl

@@ -60,6 +60,7 @@
 	{{end}}
 	<div id="diff-container">
 		{{if $showFileTree}}
+			{{$.FileIconPoolHTML}}
 			<div id="diff-file-tree" class="tw-hidden not-mobile"></div>
 			<script>
 				if (diffTreeVisible) document.getElementById('diff-file-tree').classList.remove('tw-hidden');