Simplify interface of get_distgit_files
    
    Funtion get_distgit_files is responsible for fetching files from a
    remote repository. Therefor, it requres a repository URL as an argument
    rather than accepting relative argumnets and redirecting to
    get_distgit_url to construct the URL by itself. This change is also
    easier for testing get_distgit_files.
    
    Signed-off-by: Chenxiong Qi <cqi@redhat.com>