Don't open new page for ext wiki on same repository (#20725)

- When the external wiki has been set to a file on the repository, don't
open the page on a tab.
- Resolves #20657

modules/templates/helper.go

@@ -453,6 +453,7 @@ func NewFuncMap() []template.FuncMap {
 			}
 			return items
 		},
+		"HasPrefix": strings.HasPrefix,
 	}}
 }