Switching default dpkg-source compressor for V2+ formats to xz

[Guillem Jover <guillem@debian.org>]

Hi!

I'd like to switch the default dpkg-source compressor to xz for V2+
(not for V1) source formats, as suggested by Ansgar Burchardt in [0].

  [0] <http://lists.debian.org/debian-devel/2013/05/msg00377.html>

After having switched the default dpkg-deb compressor to xz in 1.17.0,
it only makes sense to update the new source formats too, more so when
an increasing number of packages are getting switched manually. And
given that there should be less of an issue wrt compatibility with
other systems, compared to .deb packages.

So if there's no strong opposition, this would probably happen around
dpkg 1.17.3 or thereabouts.

Thanks,
Guillem