appview: pulls: fix commit links in format-patch submissions

the conditionals here were a bit confusing, branch-based PRs just
skipped commit links altogether.

appview/pages/templates/repo/pulls/pull.html

···
                <div class="flex items-center gap-2">
                  {{ i "git-commit-horizontal" "w-4 h-4" }}
                  <div class="text-sm text-gray-500 dark:text-gray-400">
-                   {{ if not $.Pull.IsPatchBased }}
-                     {{ $fullRepo := $.RepoInfo.FullName }}
-                     {{ if $.Pull.IsForkBased }}
-                        {{ if $.Pull.PullSource.Repo }}
-                          {{ $fullRepo = printf "%s/%s" $owner $.Pull.PullSource.Repo.Name }}
-                          <a href="/{{ $fullRepo }}/commit/{{ .SHA }}" class="font-mono text-gray-500 dark:text-gray-400">{{ slice .SHA 0 8 }}</a>
-                        {{ else }}
-                          <span class="font-mono">{{ slice .SHA 0 8 }}</span>
-                        {{ end }}
-                     {{ end }}
+                   <!-- attempt to resolve $fullRepo: this is possible only on non-deleted forks and branches -->
+                   {{ $fullRepo := "" }}
+                   {{ if and $.Pull.IsForkBased $.Pull.PullSource.Repo }}
+                     {{ $fullRepo = printf "%s/%s" $owner $.Pull.PullSource.Repo.Name }}
+                   {{ else if $.Pull.IsBranchBased }}
+                     {{ $fullRepo = $.RepoInfo.FullName }}
+                   {{ end }}
+
+                   <!-- if $fullRepo was resolved, link to it, otherwise just span without a link -->
+                   {{ if $fullRepo }}
+                     <a href="/{{ $fullRepo }}/commit/{{ .SHA }}" class="font-mono text-gray-500 dark:text-gray-400">{{ slice .SHA 0 8 }}</a>
                    {{ else }}
                      <span class="font-mono">{{ slice .SHA 0 8 }}</span>
                    {{ end }}