makerpms.sh: make git checkout optional
    
    In case someone is using the script from tarball, outside of git,
    the git checkout command shouldn't fail the script.
    
    Related https://pagure.io/freeipa/issue/6605
    
    Signed-off-by: Tomas Krizek <tkrizek@redhat.com>
    Reviewed-By: Pavel Vomacka <pvomacka@redhat.com>