ln: add the --relative option

With the "--relative --symbolic" options, ln computes the relative
symbolic link for the user.

So, ln works just as cp, but creates relative symbolic links instead
of copying the file.

I miss this feature since the beginning of using ln.

$ tree ./
/
`-- usr
    |-- bin
    `-- lib
        `-- foo
            `-- foo

4 directories, 1 file

$ ln -s -v --relative usr/lib/foo/foo usr/bin/foo
‘usr/bin/foo’ -> ‘../lib/foo/foo’

$ tree ./
/
`-- usr
    |-- bin
    |   `-- foo -> ../lib/foo/foo
    `-- lib
        `-- foo
            `-- foo

4 directories, 2 files

$ ln -s -v --relative usr/bin/foo usr/lib/foo/link-to-foo
‘usr/lib/foo/link-to-foo’ -> ‘foo’

$ tree ./
/
`-- usr
    |-- bin
    |   `-- foo -> ../lib/foo/foo
    `-- lib
        `-- foo
            |-- link-to-foo -> foo
            `-- foo

4 directories, 3 files

* src/Makefile.am: Reference the relpath module.
* src/ln.c (usage): Mention the new option.
(do_link): Call the relative conversion if specified.
(convert_abs_rel): Perform the relative conversion
using the relpath module.
* tests/ln/relative: Add a new test.
* tests/Makefile.am: Reference the new test.
* doc/coreutils.texi: Document the new feature.
* NEWS: Mention the new feature.

1 files changed, 32 insertions, 0 deletions

diff --git a/tests/ln/relative b/tests/ln/relative
new file mode 100755
index 000000000..cfc34691e
--- /dev/null
+++ b/tests/ln/relative
@@ -0,0 +1,32 @@
+#!/bin/sh
+# Test "ln --relative".
+
+# Copyright (C) 2012 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 of the License, 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/>.
+
+. "${srcdir=.}/init.sh"; path_prepend_ ../src
+print_ver_ ln
+
+mkdir -p usr/bin || framework_failure_
+mkdir -p usr/lib/foo || framework_failure_
+touch usr/lib/foo/foo || framework_failure_
+
+ln -sr usr/lib/foo/foo usr/bin/foo
+test $(readlink usr/bin/foo) = '../lib/foo/foo' || fail=1
+
+ln -sr usr/bin/foo usr/lib/foo/link-to-foo
+test $(readlink usr/lib/foo/link-to-foo) = 'foo' || fail=1
+
+Exit $fail