prevu can't backport 3.0 (quilt) format packages from current directory

Binary package hint: prevu

prevu can't backport 3.0 (quilt) format packages (see the dpkg-source(1) man page):

$ dget -u 'http://archive.ubuntu.com/ubuntu/pool/main/g/gdm/gdm_2.30.5-0ubuntu4.dsc'
$ cd gdm-2.30.5
$ prevu
...
dpkg-source: error: can't build with source format '3.0 (quilt)': no orig.tar file found

The problem is that the orig tarball (*.orig.tar(|.gz|.bz2)) isn't copied to the working path, so dpkg-source can't find it.

Attached is one possible way to fix it. The patch is somewhat hackish -- it'd be better if it was done using clean Python code instead of shell code.