gnucompability(chgrp): using `:1000` as a group will get accepted by uutils one but will be rejected by gnu one

[HackingRepo]

hi, uutils mianteners

i want let you known that compability issue i triggered it by using

relunsec@relunsec:~/software/coreutils/target/debug$ ./chgrp --from $'relunsec' $':1000' a
relunsec@relunsec:~/software/coreutils/target/debug$ ls -l a
-rw-rw-r-- 1 relunsec relunsec 0 Jun 11 15:31 a

while the gnu one do not accept that syntax

relunsec@relunsec:~/software/coreutils/target/debug$ gnuchgrp --from $'relunsec' $':1000' a
gnuchgrp: invalid group: ‘:1000’

that is a not a bug but is a compability issue, the uutils chgrp needs be compatible with gnu one