Merge: file: Fix `join_path` to handle paths ending with a slash.

That way, we no longer need to manually remove the trailing slash before calling `join_path`.

Pull-Request: #833
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>