When a downloaded ref doesn't contain a sha1, the error message displays
a random sha1 because of uninitialized memory. This happens when cloning
a repository that is already a clone of another one, in which case
refs/remotes/origin/HEAD is a symref.
Signed-off-by: Mike Hommey <mh@glandium.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>