Fix help output for "rpm-ostree compose"
Fix help output for "rpm-ostree compose"

"rpm-ostree compose {tree,sign} --help" acted as if no subcommand was
given.  But the compose subcommands each have a good number of options
so it's particularly important to list them.

giuseppe/rpm-ostree,mdshuai/rpm-ostree,giuseppe/rpm-ostree,mdshuai/rpm-ostree,tmp-user/rpm-ostree,mdshuai/rpm-ostree,tmp-user/rpm-ostree,tmp-user/rpm-ostree,giuseppe/rpm-ostree,tmp-user/rpm-ostree,mdshuai/rpm-ostree,giuseppe/rpm-ostree