chown,chgrp: output the correct ownership in -v messages

chown,chgrp: output the correct ownership in -v messages

* src/chown_core.c (describe_change): Accept the ownership of
the original file and output that when not changing.
This is significant when --from is specified as then
the original and specified ownership may be different.
(user_group_str): A new helper function refactored from
describe_change().
(change_file_owner): Pass the original user and group
strings to describe_change().
* test/chown/basic: Add a test case.
* NEWS: Mention the fix.