Add 'topdir' to all fedmsg/notifier messages.

This will be useful for fedmsg recipients so they can construct URLs
back to the compose.  They may need to know the compose topdir to
distinguish between https://kojipkgs.fedoraproject.org/compose/rawhide/
and https://kojipkgs.fedoraproject.org/compose/f24/

Signed-off-by: Ralph Bean <rbean@redhat.com>

pungi/notifier.py

@@ -33,6 +33,7 @@ class PungiNotifier(object):
     def _update_args(self, data):
         """Add compose related information to the data."""
         data.setdefault('compose_id', self.compose.compose_id)
+        data.setdefault('topdir', self.compose.paths.compose.topdir())
 
     def send(self, msg, **kwargs):
         """Send a message.