mv: never unlink a destination file before calling rename

mv: never unlink a destination file before calling rename

While cp --preserve=links must unlink certain destination files,
mv must never do that.
* src/copy.c (copy_internal): Pull the '! x->move_mode' test "up",
so it affects the entire condition, and not just DEREF_NEVER mode.
Reported by James Ralston in <http://bugzilla.redhat.com/438076>.
* tests/mv/atomic2: New file.  Test for the above fix.
* tests/mv/Makefile.am (TESTS): Add atomic2.
* NEWS: Mention the bug-fix.
[Bug introduced in 367719ba5f1dbd5e2f7fa2466c441f23f66a7c9e]