dgit: split brain reorg: Check $split_brain against $do_split_brain

dgit: split brain reorg: Check $split_brain against $do_split_brain

I think by now, $do_split_brain is always set early enough that
$split_brain here implies $do_split_brain.  And if $split_brain were
not set but $do_split_brain was, that would mean we hadn't actually
done the necessary work (constructing the and switching to the
dgit-view branch in the playtree or computing $dgithead).

Double check this with a couple of calls to confess.

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