debian/.gitignore: Unignore files generated by rules control

debian/.gitignore: Unignore files generated by rules control

We are going to commit to git all the files generated by
  debian/rules debian/control

This makes the git tree have a control file and therefore it is
directly buildable.  (It also avoids gbp pq producing an error message
which invoked by git-debrebase convert-from-gbp, which we are going to
use to convert the branch to git-debrebase format.)

The templating here is overkill.  Eventually, if we are lucky, we will
be able to reduce this to just debian/control.

In particular:

  * Rather than a pile of autogenerated rules in rules.gen,
    we could have suitable pattern rules, or make macros.

  * The files like xen-hypervisor-4.11-amd64.postinst could
    be generated by the rules in a hook.  Then they will
    want to be ignored again.  But they wouldn't hang off
    debian/rules debian/control.

  * The only thing that actually needs some kind of automated
    assistance, and which needs to be in the source package, is the
    binary packaage names, and dependencies, in debian/control.
    We could provide a script to update this in place, maybe, and do
    away with debian/templates/control.*.in entirely.

But for now we want control to be in git so it's easy to find, and so
that our source packages and git trees are identical as dgit requires.

Signed-off-by: Ian Jackson <ian.jackson@citrix.com>