Also specify separate project factory for project with filled repo

It uses params[:id] instead of request.fullpath.
It should fix #3132.

Files and Commits render a 404 when running with relative_url_root.

Let's assume your path is = "project/tree/master/This%20Is%20valid"
In this case gitlab renders a 404.

To fix this we should decode the path so that it looks like
"project/tree/master/This Is valid"

Update docs a bit