Add basic framework for web based repository access

/p/$project/source/repo/ is assigned to a method that
takes care of providing repository access.
For now, this results in an exception on all SCMs.

src/IDF/Scm.php

@@ -497,5 +497,10 @@ class IDF_Scm
     {
         return 0;
     }
+
+    public function repository($request, $match)
+    {
+        throw new Exception('This repository does not support web based repository access');
+    }
 }