mv: fix data loss with repeated source dir and same destination

mv: fix data loss with repeated source dir and same destination

commit v8.23-31-g90aa291 failed to consider this case,
where the previous rename has failed, thus causing the
following to remove the specified directory:

  mv dir dir dir

* src/copy.c (copy_internal): Assume this rename attempt has
succeeded, as a previous failure will already have been handled,
and we don't want to remove the source directory in this case.
* tests/cp/duplicate-sources.sh: Consolidate this test file to...
* tests/mv/dup-source.sh: ...here.  Add test cases for same
source and dest.
* tests/local.mk: Remove the consolidated test.
* NEWS: Mention the bug fix.

Reported at https://bugzilla.redhat.com/1297464