Fix diff UI for unexpandable items (#22700)

Follows #21094

Before:

There are 2 problems:

1. Sometimes, the header starts with a number, sometimes, it starts with
an icon button. It makes the UI look like misaligned.
2. The second item's bottom border is too thick (actually, that's an
empty element with border, which should be hidden as well)
3. (An old problem) the number is not mono-font


![image](https://user-images.githubusercontent.com/2114189/215935944-003fe2d3-69bf-413c-bbae-0a4668a508c3.png)


After:

Fix above problems.


![image](https://user-images.githubusercontent.com/2114189/215944811-b867a20c-110c-47a2-aa52-572a8162a44d.png)

---------

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: John Olheiser <john.olheiser@gmail.com>

templates/repo/diff/box.tmpl

@@ -76,19 +76,18 @@
 						{{$isImage := or (call $.IsBlobAnImage $blobBase) (call $.IsBlobAnImage $blobHead)}}
 						{{$isCsv := (call $.IsCsvFile $file)}}
 						{{$showFileViewToggle := or $isImage (and (not $file.IsIncomplete) $isCsv)}}
-						<div class="diff-file-box diff-box file-content {{TabSizeClass $.Editorconfig $file.Name}} mt-3" id="diff-{{$file.NameHash}}" data-old-filename="{{$file.OldName}}" data-new-filename="{{$file.Name}}" {{if $file.ShouldBeHidden}}data-folded="true"{{end}}>
+						{{$isExpandable := or (gt $file.Addition 0) (gt $file.Deletion 0) $file.IsBin}}
+						<div class="diff-file-box diff-box file-content {{TabSizeClass $.Editorconfig $file.Name}} mt-3" id="diff-{{$file.NameHash}}" data-old-filename="{{$file.OldName}}" data-new-filename="{{$file.Name}}" {{if or ($file.ShouldBeHidden) (not $isExpandable)}}data-folded="true"{{end}}>
 							<h4 class="diff-file-header sticky-2nd-row ui top attached normal header df ac sb">
 								<div class="df ac">
-									{{if or (gt $file.Addition 0) (gt $file.Deletion 0) $file.IsBin}}
-										<a role="button" class="fold-file muted mr-2">
-											{{if $file.ShouldBeHidden}}
-												{{svg "octicon-chevron-right" 18}}
-											{{else}}
-												{{svg "octicon-chevron-down" 18}}
-											{{end}}
-										</a>
-									{{end}}
-									<div class="bold df ac">
+									<a role="button" class="fold-file muted mr-2" {{if not $isExpandable}}style="visibility: hidden"{{end}}>
+										{{if $file.ShouldBeHidden}}
+											{{svg "octicon-chevron-right" 18}}
+										{{else}}
+											{{svg "octicon-chevron-down" 18}}
+										{{end}}
+									</a>
+									<div class="bold df ac mono">
 										{{if $file.IsBin}}
 											<span class="ml-1 mr-3">
 												{{$.locale.Tr "repo.diff.bin"}}