dpkg-source/3.0 (quilt): allow /dev/null in automatic patch

dpkg-source/3.0 (quilt): allow /dev/null in automatic patch

* scripts/Dpkg/Source/Patch.pm (add_diff_directory): If option
use_dev_null is set, then the label of the "old" file will be
'/dev/null' if we're creating a new file (instead of using the
name of the created file, like it's currently done in .diff
of version "1.0" source packages).
* scripts/Dpkg/Source/Package/V2.pm (prepare_build): Use the new
option "use_dev_null" when creating the automatic patch.