This script can be used as a notification handler. For most messages it
does nothing, but when it sees a new commit in an ostree repo, it will
wait for a signature of the new commit to appear.
This is useful for building images later so that they include the
signature as well.
Fixes: https://pagure.io/pungi/issue/650
Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>