dgit: Option parsing: Fix distro/suite reference bugs

By moving parseopts_late_defaults call.

This is conceptually the same as finalise_late_opts, so do it there.
We have to add a missing call to build_prep_early, as otherwise we
call massage_dbp_args too soon.

Also move a nopushing() call until after $isuite is set

All of this means that we now honour $isuite (and sometimes $idistro)
much better.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>