I just realised that this change avoids a more problematic issue.
The loss of files on case insensitive file systems supporting hardlinks.
Consider hfs...

  $ truncate -s1G hfs.img
  $ mkfs.hfsplus hfs.img
  $ mkdir hfs
  $ sudo mount hfs.img hfs
  $ sudo su
  # cd hfs
  # touch foo
  # ln foo blah
  # mv foo Foo  # foo is removed!
  # ls -l
  -rw-r--r--. 1 root root 0 Nov 22 02:42 blah

In the attached patch I've added a test case,
removed an old test case that pondered the issue in 5.0.91?,
updated NEWS and mentioned the issue in copy.c

cheers,
Pádraig.