Merge branch 'hx/push-atomic-with-cert'

Merge branch 'hx/push-atomic-with-cert'

"git push" that wants to be atomic and wants to send push
certificate learned not to prepare and sign the push certificate
when it fails the local check (hence due to atomicity it is known
that no certificate is needed).

* hx/push-atomic-with-cert:
  send-pack: run GPG after atomic push checking