scripts/release_helper.sh: manually build debian/changelog during autobuilds, improve...

The rebasing of the autobuild tree against master can be annoying.
Therefore create a temporary branch that's been thrown away after
building was successfull.

Thanks to Axel Beckert <abe@deuxchevaux.org> and his script
http://repo.or.cz/w/conkeror.git/blob/HEAD:/contrib/debian/nightlybuild.sh
for some inspiration regarding building debian/changelog.