Simplify test cruft-empty-diff; use only templates; no copies in ./orig.

The test 'cruft-empty-diff' replicated all standard templates in
./orig. Here we instead use the script ./pre_upstream to copy all
filled Debian files into the package directory before the orig.tar.gz
is created. It also renders the diff empty, but is probably more fool-
and future-proof.

Signed-off-by: Chris Lamb <lamby@debian.org>