Always show owner/repo name in compare page dropdowns (#37172) (#37200)

Backport #37172 by @xingxing21

Fixes: https://github.com/go-gitea/gitea/issues/36677

The fix is a template-only change in
[templates/repo/diff/compare.tmpl:16-25](vscode-webview://1ca9j6f1e3qtaf59o0cr4ind65ulf8mevvbbbq88int1gg2lncar/templates/repo/diff/compare.tmpl#L16-L25).

Before, the display names were built with conditional logic:

All four variables defaulted to just the owner name (Examples)
$HeadCompareName was only upgraded to owner/repo format in two narrow
cases:
Same org, different repos
A root repo exists with the same owner as the head repo
All other cases (same-repo PRs, cross-org PRs) got owner-only labels
After, all four variables are unconditionally set to owner/repo format
using printf "%s/%s":

```
- {{$BaseCompareName := printf "%s/%s" $.BaseName $.Repository.Name -}}
- {{- $HeadCompareName := printf "%s/%s" $.HeadRepo.OwnerName $.HeadRepo.Name -}}
- {{- $OwnForkCompareName := "" -}}
- {{- if .OwnForkRepo -}}
-	{{- $OwnForkCompareName = printf "%s/%s" .OwnForkRepo.OwnerName .OwnForkRepo.Name -}}
- {{- end -}}
- {{- $RootRepoCompareName := "" -}}
- {{- if .RootRepo -}}
-	{{- $RootRepoCompareName = printf "%s/%s" .RootRepo.OwnerName .RootRepo.Name -}}
- {{- end -}}

+ {{$BaseCompareName := $.Repository.FullName -}}
+ {{$HeadCompareName := $.HeadRepo.FullName -}}
+ {{$OwnForkCompareName := "" -}}
+ {{if $.OwnForkRepo -}}
+	{{$OwnForkCompareName = $.OwnForkRepo.FullName -}}
+ {{end -}}
+ {{$RootRepoCompareName := "" -}}
+ {{if $.RootRepo -}}
+	{{$RootRepoCompareName = $.RootRepo.FullName -}}
+ {{end -}}
```

These variables drive the labels in the base and head branch selector
buttons and their dropdown items. No backend changes were needed —
$.BaseName, $.Repository.Name, $.HeadRepo.OwnerName, and $.HeadRepo.Name
were already available in the template context.

Co-authored-by: Xing Hong <39619359+xingxing21@users.noreply.github.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

templates/repo/diff/compare.tmpl

@@ -13,22 +13,16 @@
 				{{ctx.Locale.Tr "action.compare_commits_general"}}
 			{{end}}
 		</h2>
-		{{$BaseCompareName := $.BaseName -}}
-		{{- $HeadCompareName := $.HeadRepo.OwnerName -}}
-		{{- if and (eq $.BaseName $.HeadRepo.OwnerName) (ne $.Repository.Name $.HeadRepo.Name) -}}
-			{{- $HeadCompareName = printf "%s/%s" $.HeadRepo.OwnerName $.HeadRepo.Name -}}
-		{{- end -}}
-		{{- $OwnForkCompareName := "" -}}
-		{{- if .OwnForkRepo -}}
-			{{- $OwnForkCompareName = .OwnForkRepo.OwnerName -}}
-		{{- end -}}
-		{{- $RootRepoCompareName := "" -}}
-		{{- if .RootRepo -}}
-			{{- $RootRepoCompareName = .RootRepo.OwnerName -}}
-			{{- if eq $.HeadRepo.OwnerName .RootRepo.OwnerName -}}
-				{{- $HeadCompareName = printf "%s/%s" $.HeadRepo.OwnerName $.HeadRepo.Name -}}
-			{{- end -}}
-		{{- end -}}
+		{{$BaseCompareName := $.Repository.FullName -}}
+		{{$HeadCompareName := $.HeadRepo.FullName -}}
+		{{$OwnForkCompareName := "" -}}
+		{{if $.OwnForkRepo -}}
+			{{$OwnForkCompareName = $.OwnForkRepo.FullName -}}
+		{{end -}}
+		{{$RootRepoCompareName := "" -}}
+		{{if $.RootRepo -}}
+			{{$RootRepoCompareName = $.RootRepo.FullName -}}
+		{{end -}}
 
 		<div class="ui segment choose branch">
 			<a class="tw-mr-2" href="{{$.HeadRepo.Link}}/compare/{{PathEscapeSegments $.HeadBranch}}{{$compareSeparator}}{{if not $.PullRequestCtx.SameRepo}}{{PathEscape $.BaseName}}/{{PathEscape $.Repository.Name}}:{{end}}{{PathEscapeSegments $.BaseBranch}}" title="{{ctx.Locale.Tr "repo.pulls.switch_head_and_base"}}">{{svg "octicon-git-compare"}}</a>