mv: allow moving symlink onto same-inode dest with >= 2 hard links

mv: allow moving symlink onto same-inode dest with >= 2 hard links

Normally, mv detects a few subtle cases in which proceeding with a
same-file rename would, with very high probability, cause data loss.
Here, we have found a corner case in which one of these same-inode
tests makes mv refuse to perform a useful operation.  Permit that
corner case.
* src/copy.c (same_file_ok): Detect/exempt this case.
* tests/mv/symlink-onto-hardlink: New test.
* tests/Makefile.am (TESTS): Add it.
* NEWS (Bug fixes): Mention it.
Initially reported by: Matt McCutchen in http://bugs.gnu.org/6960.
Raised again by Anders Kaseorg due to http://bugs.debian.org/654596.
Improved-by: Paul Eggert.