Fix broken code editor diff preview (#23307)

Close #23265, the code editor diff preview has been broken for long
time.

* Fix the regression for `data-line-num`
    * `.code-diff` is necessary to show the line number
* Fix the regression for #12434
* The diff:
[12434](https://github.com/go-gitea/gitea/pull/12434/files?diff=unified&w=1)
    * It hides the Type(4) (aka HunkHeader)  for unexpected cases.


Diff with ignoring whitespaces:
https://github.com/go-gitea/gitea/pull/23307/files?diff=unified&w=1

Before: see the issue #23265

After:

![image](https://user-images.githubusercontent.com/2114189/222942810-286dc9af-0b39-4e9d-8585-8c299b881241.png)

templates/repo/editor/diff_preview.tmpl

@@ -1,6 +1,6 @@
 <div class="diff-file-box">
 	<div class="ui attached table segment">
-		<div class="file-body file-code code-view code-diff-unified unicode-escaped">
+		<div class="file-body file-code code-diff code-diff-unified unicode-escaped">
 			<table>
 				<tbody>
 					{{template "repo/diff/section_unified" dict "file" .File "root" $}}