Fix code tab link when viewing tags (#3908)

Signed-off-by: Ambrose Chua <ambrose@chua.family>

templates/repo/header.tmpl

@@ -48,7 +48,7 @@
 	<div class="ui tabs container">
 		<div class="ui tabular stackable menu navbar">
 			{{if .Repository.UnitEnabled $.UnitTypeCode}}
-			<a class="{{if .PageIsViewCode}}active{{end}} item" href="{{.RepoLink}}{{if (ne .BranchName .Repository.DefaultBranch)}}/src/branch/{{.BranchName}}{{end}}">
+			<a class="{{if .PageIsViewCode}}active{{end}} item" href="{{.RepoLink}}{{if (ne .BranchName .Repository.DefaultBranch)}}/src/{{.BranchNameSubURL}}{{end}}">
 				<i class="octicon octicon-code"></i> {{.i18n.Tr "repo.code"}}
 			</a>
 			{{end}}