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>

services/gitdiff/gitdiff.go

@@ -474,7 +474,7 @@ func ParsePatch(maxLines, maxLineCharacters, maxFiles int, reader io.Reader, ski
 	sb := strings.Builder{}
 
 	// OK let's set a reasonable buffer size.
-	// This should be let's say at least the size of maxLineCharacters or 4096 whichever is larger.
+	// This should be at least the size of maxLineCharacters or 4096 whichever is larger.
 	readerSize := maxLineCharacters
 	if readerSize < 4096 {
 		readerSize = 4096

templates/repo/diff/section_unified.tmpl

@@ -1,9 +1,9 @@
 {{$file := .file}}
 {{range $j, $section := $file.Sections}}
 	{{range $k, $line := $section.Lines}}
-		{{if or $.root.AfterCommitID (ne .GetType 4)}}
 		<tr class="{{DiffLineTypeToStr .GetType}}-code nl-{{$k}} ol-{{$k}}" data-line-type="{{DiffLineTypeToStr .GetType}}">
 			{{if eq .GetType 4}}
+				{{if $.root.AfterCommitID}}
 					<td colspan="2" class="lines-num">
 						{{if or (eq $line.GetExpandDirection 3) (eq $line.GetExpandDirection 5)}}
 							<a role="button" class="blob-excerpt" data-url="{{$.root.RepoLink}}/blob_excerpt/{{PathEscape $.root.AfterCommitID}}" data-query="{{$line.GetBlobExcerptQuery}}&style=unified&direction=down&wiki={{$.root.PageIsWiki}}" data-anchor="diff-{{$file.NameHash}}K{{$line.SectionInfo.RightIdx}}">
@@ -21,6 +21,10 @@
 							</a>
 						{{end}}
 					</td>
+				{{else}}
+					{{/* for code file preview page or comment diffs on pull comment pages, do not show the expansion arrows */}}
+					<td colspan="2" class="lines-num"></td>
+				{{end}}
 			{{else}}
 				<td class="lines-num lines-num-old" data-line-num="{{if $line.LeftIdx}}{{$line.LeftIdx}}{{end}}"><span rel="{{if $line.LeftIdx}}diff-{{$file.NameHash}}L{{$line.LeftIdx}}{{end}}"></span></td>
 				<td class="lines-num lines-num-new" data-line-num="{{if $line.RightIdx}}{{$line.RightIdx}}{{end}}"><span rel="{{if $line.RightIdx}}diff-{{$file.NameHash}}R{{$line.RightIdx}}{{end}}"></span></td>
@@ -52,5 +56,4 @@
 			</tr>
 		{{end}}
 	{{end}}
-	{{end}}
 {{end}}

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" $}}