cp: when copying hardlink over itself, the backup should also be a hardlink

[BenWiederhake]

When copying hardlink over itself, the backup should also be a hardlink, and its creation announced if -v was given.

$ rm -f README.md~ && cp -vlbf README.md README.md && ls -1i README.md*
'README.md' -> 'README.md~'
20092600 README.md
20092600 README.md~
$ rm -f README.md~ && cargo run -q cp -vlbf README.md README.md && ls -1i README.md*
20092600 README.md
20074037 README.md~

Found while reading #6496, but only remotely related.