Call "gpg" using the right API when validating the signature on
tags.

* mg/gpg-interface-using-status:
  pretty: make %GK output the signing key for signed commits
  pretty: parse the gpg status lines rather than the output
  gpg_interface: allow to request status return
  log-tree: rely upon the check in the gpg_interface
  gpg-interface: check good signature in a reliable way