mv: use reflink=auto mode by default

mv: use reflink=auto mode by default

On some filesystems (BTRFS), moving a file within the filesystem may
cross subvolume boundaries and we can use a lightweight reflink copy,
similar to what cp(1) can do, which is faster than a full file copy.
This is enabled by default because it's only an optimization for
the fall back copy and does not break user expectations or usability.

* src/mv.c (cp_option_init): Set the reflink mode to AUTO.
* NEWS: Mention the improvement.