chgrp: verbose "retained as" may print an empty group name for unmapped GIDs

Component

chgrp

Description

When running in group-only mode (e.g., chgrp) with --verbose and the requested group equals the current group (so no change occurs), wrap_chown should prints

group of 'FILE' retained as <group>

however

    } else if verbosity.level == VerbosityLevel::Verbose {
        out = if verbosity.groups_only {
            format!(
                "group of {} retained as {}",
                path.quote(),
                entries::gid2grp(dest_gid).unwrap_or_default()
            )
        } else {
            format!(
                "ownership of {} retained as {}:{}",
                path.quote(),
                entries::uid2usr(dest_uid).unwrap_or_else(|_| dest_uid.to_string()),
                entries::gid2grp(dest_gid).unwrap_or_else(|_| dest_gid.to_string())
            )
        };
    }

will print out empty string while the C version prints group id.

Test / Reproduction Steps

core_sbx$ touch tmp
core_sbx$ chgrp 1337 tmp
core_sbx$ chgrp -v 1337 tmp                  
group of 'tmp' retained as 1337
core_sbx$ ../../coreutils/target/release/chgrp -v 1337 tmp
../../coreutils/target/release/chgrp: group of 'tmp' retained as 

Impact

Incorrect or confusing diagnostics. Breaks tests or scripts that parse verbose output.

Start in chgrp's wrap_chown implementation, specifically the verbose, group-only retained-as branch. Reproduce the case with a numeric unmapped GID such as 1337, then compare the Rust output with the expected C-style diagnostic. Done means the verbose message includes the numeric GID instead of an empty group name.