Use xz compression by default

In pristine-tar branch, there is tar.xz pristine-tar commit instead of tar.gz.

Without this fix, gbp will raise the following error.

  gbp:info: Creating
  /home/kenhys/work/debian/workspace/epics/build-area/epics-base_7.0.8+dfsg1.orig.tar.gz
  gbp:error: Error creating epics-base_7.0.8+dfsg1.orig.tar.gz:
  Pristine-tar couldn't checkout "epics-base_7.0.8+dfsg1.orig.tar.gz":
  fatal: path 'epics-base_7.0.8+dfsg1.orig.tar.gz.delta' does not exist
  in 'refs/heads/pristine-tar' pristine-tar: git show
  refs/heads/pristine-tar:epics-base_7.0.8+dfsg1.orig.tar.gz.delta
  failed

Signed-off-by: Kentaro Hayashi kenhys@gmail.com