remove unused call to $.HeadRepo in view_title template (#32317)

This is only populated in
[`ParseCompareInfo`](https://github.com/search?q=repo%3Ago-gitea%2Fgitea%20%20.Data%5B%22HeadRepo%22%5D&type=code)
which is called in two handlers:

*
[`CompareAndPullRequestPost`](9206fbb55f/routers/web/repo/pull.go (L1246))
- a JSON post handler that doesn't render templates
*
[`CompareDiff`](9206fbb55f/routers/web/repo/compare.go (L706))
- which can render `diff/box.tmpl` and `diff/compare.tmpl`

(cherry picked from commit 1cd3f698591edf4fba7880a150b05855cdf40d47)

templates/repo/issue/view_title.tmpl

@@ -117,7 +117,7 @@
 										{{$sameBase := ne $.BaseName $.HeadUserName}}
 										{{$differentBranch := ne . $.HeadBranch}}
 										{{if or $sameBase $differentBranch}}
-											<div class="item {{if eq $.BaseBranch .}}selected{{end}}" data-branch="{{.}}">{{$.BaseName}}{{if $.HeadRepo}}/{{$.HeadRepo}}{{end}}:{{.}}</div>
+											<div class="item {{if eq $.BaseBranch .}}selected{{end}}" data-branch="{{.}}">{{$.BaseName}}:{{.}}</div>
 										{{end}}
 									{{end}}
 								</div>