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'.

tests/guix-git-authenticate.sh

@@ -0,0 +1,56 @@
+# 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/>.
+
+#
+# Test the 'guix git authenticate' command-line utility.
+#
+
+# Skip if we're not in a Git checkout.
+[ -d "$abs_top_srcdir/.git" ] || exit 77
+
+# Skip if there's no 'keyring' branch.
+guile -c '(use-modules (git))
+  (member "refs/heads/keyring" (branch-list (repository-open ".")))' || \
+    exit 77
+
+# Keep in sync with '%default-channels' in (guix channels)!
+intro_commit="9edb3f66fd807b096b48283debdcddccfea34bad"
+intro_signer="BBB0 2DDF 2CEA F6A8 0D1D  E643 A2A0 6DF2 A33A 54FA"
+
+cache_key="test-$$"
+
+guix git authenticate "$intro_commit" "$intro_signer"	\
+     --cache-key="$cache_key" --stats			\
+     --end=9549f0283a78fe36f2d4ff2a04ef8ad6b0c02604
+
+rm "$XDG_CACHE_HOME/guix/authentication/$cache_key"
+
+# Commit and signer of the 'v1.0.0' tag.
+v1_0_0_commit="6298c3ffd9654d3231a6f25390b056483e8f407c"
+v1_0_0_signer="3CE4 6455 8A84 FDC6 9DB4  0CFB 090B 1199 3D9A EBB5" # civodul
+v1_0_1_commit="d68de958b60426798ed62797ff7c96c327a672ac"
+
+# This should fail because these commits lack '.guix-authorizations'.
+if guix git authenticate "$v1_0_0_commit" "$v1_0_0_signer" \
+	--cache-key="$cache_key" --end="$v1_0_1_commit";
+then false; else true; fi
+
+# This should work thanks to '--historical-authorizations'.
+guix git authenticate "$v1_0_0_commit" "$v1_0_0_signer" 	\
+     --cache-key="$cache_key" --end="$v1_0_1_commit" --stats	\
+     --historical-authorizations="$abs_top_srcdir/etc/historical-authorizations"