Make sure all commits have a proper subject
    
    Until the first empty line, Git will always consider all of the commit
    message to be part of the subject line. When generating the clog from
    the RPM changelog, this means that all lines in the changelog end up as
    part of the subject.
    
    This patch reworks the clog() function to always have a distinct subject
    line, either by accepting the `-m` argument or by treating the first
    line of the changelog as special and inserting an extra newline for it.
    
    Merges: https://pagure.io/rpkg/pull-request/562
    
    Signed-off-by: Stephen Gallagher <sgallagh@redhat.com>