Choose SHA1 sums more intelligently

By checking the actual gitbuilder archive that we care about, if
--distro is passed. If it is not, tweak the default to 'rhel' instead of
'ubuntu' since that seems to produce better results. To be clear, this
is a workaround I'm putting in place since our gitbuilders never seem to
be in sync.

Signed-off-by: Zack Cerza <zack@redhat.com>