Use File::Spec->abs2rel() instead of home-grown relative_path

which actually gets some inputs wrong (e.g. "/" relative to "/")

t/relative_path.t

@@ -1,41 +0,0 @@
-#!/usr/local/bin/perl
-
-#
-# Testing relative_path();
-#
-
-# load as a library
-BEGIN { use lib qw(. ..); require "stow"; }
-
-use Test::More tests => 5;
-
-is(
-    relative_path('a/b/c', 'a/b/d'),
-    '../d',
-    => 'different branches'
-);
-
-is(
-    relative_path('/a/b/c', '/a/b/c/d'),
-    'd',
-    => 'lower same branch'
-);
-
-is(
-    relative_path('a/b/c', 'a/b'),
-    '..',
-    => 'higher, same branch'
-);
-
-is(
-    relative_path('/a/b/c', '/d/e/f'),
-    '../../../d/e/f',
-    => 'common parent is /'
-);
-
-is(
-    relative_path('///a//b//c////', '/a////b/c/d////'),
-    'd',
-    => 'extra /\'s '
-);
-