Depend on newer devscripts

These contain a uscan that handles repackaging at the right level and
won't let us pick up the wrong tarball name.

Closes: #635920
Thanks: gregor herrmann and all others involved
Avoid backtrace on config file parse errors

"gbp buildpackage" did this already but others didn't

Closes: #733759
Make parsing config file sections symmetric

Always read the legacy command's config file section prior to the
subcommand's config file section.

Until now 'gbp <subcommand>' would read '[subcommand]' as well as
'[gbp-<subcommand>]' sections while 'gbp-<subcommand>' would only read
'[gbp-<subcommand>]' sections.

Closes: #733759
Change the default of cleaner to /bin/true

Running a clean command within version control by default nowadays
causes more trouble than it helps. It's unnecessary with
pbuilder/cowbuilder/sbuild and with export-dir. So change the default
from 'debuild clean' to a noop ('/bin/true').

Closes: #670624