Add 'guix git authenticate'.

* guix/scripts/git.scm, guix/scripts/git/authenticate.scm,
tests/guix-git-authenticate.sh: New files.
* Makefile.am (MODULES): Add the *.scm files.
(SH_TESTS): Add 'tests/guix-git-authenticate.sh'.
* doc/guix.texi (Channels)[Specifying Channel Authorizations]: Mention
'guix git authenticate'.
(Invoking guix git authenticate): New node.
* po/guix/POTFILES.in: Add 'guix/scripts/git.scm' and
'guix/scripts/git/authenticate.scm'.

guix/scripts/git.scm

@@ -0,0 +1,63 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2020 Ludovic Courtès <ludo@gnu.org>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (guix scripts git)
+  #:use-module (ice-9 match)
+  #:use-module (guix ui)
+  #:export (guix-git))
+
+(define (show-help)
+  (display (G_ "Usage: guix git COMMAND ARGS...
+Operate on Git repositories.\n"))
+  (newline)
+  (display (G_ "The valid values for ACTION are:\n"))
+  (newline)
+  (display (G_ "\
+   authenticate    verify commit signatures and authorizations\n"))
+  (newline)
+  (display (G_ "
+  -h, --help             display this help and exit"))
+  (display (G_ "
+  -V, --version          display version information and exit"))
+  (newline)
+  (show-bug-report-information))
+
+(define %sub-commands '("authenticate"))
+
+(define (resolve-sub-command name)
+  (let ((module (resolve-interface
+                 `(guix scripts git ,(string->symbol name))))
+        (proc (string->symbol (string-append "guix-git-" name))))
+    (module-ref module proc)))
+
+(define (guix-git . args)
+  (with-error-handling
+    (match args
+      (()
+       (format (current-error-port)
+               (G_ "guix git: missing sub-command~%")))
+      ((or ("-h") ("--help"))
+       (show-help)
+       (exit 0))
+      ((or ("-V") ("--version"))
+       (show-version-and-exit "guix git"))
+      ((sub-command args ...)
+       (if (member sub-command %sub-commands)
+           (apply (resolve-sub-command sub-command) args)
+           (format (current-error-port)
+                   (G_ "guix git: invalid sub-command~%")))))))