Pretend to allow full path to backup files

This has annoyed me several times, so let's fix it.

Basically, this allows you to use tab-completion when selecting a backup to restore from.

    cd /var/www/discourse
    script/discourse restore pu<TAB>b<TAB><TAB><TAB TAB><look at list of backups>v<TAB>5<TAB><ENTER>
    # Results in
    script/discourse public/backups/default/vagrant-discourse-2014-05-15-035437.tar.gz