tests: add a case verifying mv on case insensitive file systems

tests: add a case verifying mv on case insensitive file systems

* NEWS: Update the recent entry to also mention the avoidance
of incorrectly unlinking a multi-hardlinked "source" file when
presented with source and dest that only differ in case.
* src/copy.c (same_file_ok): Mention the case issue with same_name().
* tests/mv/hardlink-case.sh: Test the issue on HFS+.
* tests/local.mk: Reference the new test case.
* tests/mv/vfat: Remove an old related but unused test case.