remove unused call to $.HeadRepo in view_title template

This is only populated in `ParseCompareInfo` which is called in two
handlers:

* `CompareAndPullRequestPost` - a JSON post handler that doesn't
    render templates
* `CompareDiff` - which can render diff/box.tmpl and diff/compare.tmpl

Author: bohde

templates/repo/issue/view_title.tmpl

@@ -102,7 +102,7 @@
 										{{$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>