Set GPGME_SIGSUM_KEY_REVOKED also for gpg.
* src/verify.c (calc_sig_summary): Handle GPG_ERR_CERT_REVOKED.
parse_new_sig() handles a revoked key by setting sig->status to
GPG_ERR_CERT_REVOKED, but then later calc_sig_summary() expects that
code in sig->validity_reason.
Additional comments added by wk.