It can now authenticate 14K+ commits in 23s instead of 4mn20.

* build-aux/git-authenticate.scm (%authorized-signing-keys): Turn
fingerprints into bytevectors.
(with-temporary-files): Remove.
(commit-signing-key): Add 'keyring' parameter.  Use
'string->openpgp-packet' and 'verify-openpgp-signature' instead of (guix
gnupg) procedures.
(authenticate-commit): Add 'keyring' parameter.  Pass it to
'commit-signing-key'.  Adjust to SIGNING-KEY being an <openpgp-public-key>.
(authenticate-commits): Remove 'parameterize'.  Load keyring with
'get-openpgp-keyring'.
(git-authenticate): When printing stats, adjust to SIGNER being an
<openpgp-public-key>.