files-backend: make sure files_rename_ref() always reach the end

This is a no-op patch. It prepares the function so that we can release
resources (to be added later in this function) before we return.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>