# $1 should be .txt file in autobuild directory, eg.:
# ftp://.../gentoo/releases/amd64/autobuilds/latest-stage3-amd64.txt
local relative prefix
relative=$(wget -qO - $1 | grep -o '^[^# ]\+')
(( $pipestatus[1] )) && die "could not fetch $1"
[[ -z "$relative" ]] && die "no filename found in $1"
prefix=$1:h
shift
fsb_download_gentoo_stage $prefix/$relative "$@"