Add (guix git-authenticate).

* build-aux/git-authenticate.scm (commit-signing-key)
(read-authorizations, commit-authorized-keys, authenticate-commit)
(load-keyring-from-blob, load-keyring-from-reference)
(authenticate-commits, authenticated-commit-cache-file)
(previously-authenticated-commits, cache-authenticated-commit): Remove.
* build-aux/git-authenticate.scm (git-authenticate): Pass
 #:default-authorizations to 'authenticate-commits'.
* guix/git-authenticate.scm: New file, with code taken from
'build-aux/git-authenticate.scm'.  Remove references to
'%historical-authorized-signing-keys' and add #:default-authorizations
parameter instead.
* Makefile.am (MODULES): Add it.
(authenticate): Depend on guix/git-authenticate.go.