ln: --relative: fix updating of existing symlinks

ln: --relative: fix updating of existing symlinks

Don't dereference an existing symlink being replaced.
I.E. generate the symlink relative to the symlink's containing dir,
rather than to some arbitrary place it points to.

* src/ln.c (convert_abs_rel): Don't consider the final component
of the symlink name when canonicalizing, as we want to avoid
dereferencing the final component.
* tests/ln/relative.sh: Add a test case.
* NEWS: Mention the fix.
Resolves http://bugs.gnu.org/14116