mv: fix invalid numbered backup path

[hamflx]

see https://github.com/uutils/coreutils/pull/6040#issuecomment-2016746488

the gnu mv:

hamflx@hamflx-workstation:~/coreutils/test$ find . -delete && mkdir C D E && touch C/c D/d E/e
hamflx@hamflx-workstation:~/coreutils/test$ mv -T --backup=numbered C E/
hamflx@hamflx-workstation:~/coreutils/test$ tree -a
.
├── D
│   └── d
├── E
│   └── c
└── E.~1~
    └── e

3 directories, 3 files

the uu_mv:

hamflx@hamflx-workstation:~/coreutils/test$ find . -delete && mkdir C D E && touch C/c D/d E/e
hamflx@hamflx-workstation:~/coreutils/test$ ../target/debug/mv -T --backup=numbered C E/
hamflx@hamflx-workstation:~/coreutils/test$ tree -a
.
├── D
│   └── d
└── E
    └── c

2 directories, 2 files

[sylvestre]

Could you please add an highlevel integration test in https://github.com/uutils/coreutils/blob/main/tests/by-util/test_mv.rs ? thanks

[hamflx]

@sylvestre I have added integration test.

[github-actions[bot]]

GNU testsuite comparison:

Skip an intermittent issue tests/rm/rm1 (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/tail/inotify-dir-recreate (passes in this run but fails in the 'main' branch)

[hamflx]

@sylvestre @tertsdiepraam Is there anything else I need to do if I want this PR to be merged?