cp: backup dest symlink linking to source

[cakebaker]

This PR covers a special case of https://github.com/uutils/coreutils/pull/5731, with the destination being a symlink to the source. So far this caused a "same file" error whereas it works fine with GNU cp.

[github-actions[bot]]

GNU testsuite comparison:

Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)