backup_control: Add todo for gnu compliant behavior
backup_control: Add todo for gnu compliant behavior

The logic behind the file-backup implementation currently doesn't comply 100%
with what the GNU manual [1] describes. Adds a TODO so it isn't forgotten.

[1]: https://www.gnu.org/software/coreutils/manual/html_node/Backup-options.html

uutils/coreutils,uutils/coreutils,uutils/coreutils