Fix broken code editor diff preview (#23307) (#23320)
Backport #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)
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>