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.