Consistently use bz2 or xz when downloading from savannah.gnu.org

[Googulator]

Savannah supports links like http://git.savannah.gnu.org/cgit/coreutils.git/snapshot/coreutils-8.32.tar.bz2 or even http://git.savannah.gnu.org/cgit/coreutils.git/snapshot/coreutils-8.32.tar.xz which provide better compression than the gz versions.