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

[GiteaBot]

Backport #37172 by @xingxing21

Fixes: #36677

The fix is a template-only change in templates/repo/diff/compare.tmpl:16-25.

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.