... for similar reasons to the previous commit about the diff(1) command.

One can't get expr's return code with "if ! expr ; then ..."

... even if that's in a "if ! diff" block and thus should be executed only
whenever diff returns !0. Go figure.

That's particularly needed when --iso is passed: we're comparing list
of packages that are built from different sources.

diff(1) returns 1 if the compared contents differ.

This will allow to purge a lot of X and GTK dependencies we don't need.

Otherwise, gnupg-agent pulls pinentry-gtk2 and a whole lot of X dependencies.

... and gracefully handle upgrades from Wheezy.

1. Handle upgrade from Wheezy by dropping obsolete APT pinning.
2. Upgrade list of dependencies for Jessie.

Also now takes two arguments: pathes to a Tails and a jenkins-jobs Tails
repo.

It's not specific to the BuildTailsISOJob class.

Move the default JOB_SNIPPET to a class constant, and make it optional to configure it via the constructor.

Set the Tails RM as job recipient only for the base branches, don't set
a recipent otherwise.

Configure the BuildTailsISOJob object on initialization.

We're tweaking it with file_line etc. instead.

I need to push syslinux there, built for both i386 (for inclusion into
the ISO) and amd64 (for builder VMs).

As discussed with him, he doesn't need these privs in practice.

Note: given the dependencies on parcimonie (that's not exactly
lightweight yet) and systemd (that's not very widespread on Debian
servers yet), it doesn't feel worth pushing this to the upstream
reprepro module yet.