tar: fix --verify option, which broke in 1.24

* NEWS: Document this.
* src/compare.c (verify_volume): Decode the header before invoking
diff_archive, as diff_archive no longer does this as of the
2010-06-28 commit.  Also, don't try to invoke diff_archive on a
zero block.
* tests/Makefile.am (TESTSUITE_AT): Add verify.at.
* tests/testsuite.at: Include verify.at.
* tests/verify.at: New file.

tests/verify.at

@@ -0,0 +1,37 @@
+# Process this file with autom4te to create testsuite. -*- Autotest -*-
+
+# Test suite for GNU tar.
+# Copyright (C) 2010 Free Software Foundation, Inc.
+
+# This program 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, or (at your option)
+# any later version.
+
+# This program 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 this program; if not, write to the Free Software
+# Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA
+# 02110-1301, USA.
+
+# Check that tar --verify works.
+# Problem reported by Gilles Espinasse in
+# <http://lists.gnu.org/archive/html/bug-tar/2010-11/msg00065.html>.
+
+AT_SETUP([verify])
+AT_KEYWORDS([verify])
+
+AT_TAR_CHECK([
+touch foo
+tar -cvf archive.tar --verify foo
+],
+[0],
+[foo
+Verify foo
+])
+
+AT_CLEANUP