Bug#648632: change default compress to xz is problematic

[Osamu Aoki]

Hi,

I withdraw:
   As I read this and think about the current situation, for the non-1.0
   format, it may be better to use xz for repack as the default mode.
   (This is todo)

Its tempting but this will break things and not simple to enable this
just for watch version=4.

Instead make recommendation to set compress=xz in manpage.

Osamu