Fix reporting of hardlink mismatch during compare

* src/common.h (quote_n_colon): New prototype.
* src/misc.c (quote_n_colon): New function.
* src/compare.c (report_difference, diff_link): Use quote_n_colon.

* tests/difflink.at: New file.
* tests/Makefile.am: Add difflink.at
* tests/testsuite.at: Likewise.

tests/difflink.at

@@ -0,0 +1,35 @@
+# This file is part of GNU tar test suite  -*- Autotest -*-
+#
+# Copyright 2017 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, see <http://www.gnu.org/licenses/>.
+AT_SETUP([link mismatch])
+AT_KEYWORDS([diff difflink])
+AT_TAR_CHECK([
+mkdir a
+genfile -f a/x
+ln -s x a/y
+ln a/y a/z
+tar cf a.tar a
+rm a/z
+ln -s x a/z
+tar df a.tar
+],
+[1],
+[a/z: Not linked to a/y
+],
+[],
+[],[],[ustar]) # Testing one format is enough
+AT_CLEANUP
+