dgit: split brain reorg: Move do_split_brain setting

dgit: split brain reorg: Move do_split_brain setting

We have to introduce a new "time", build_or_push_prep_modes, which is
the point at which we know we have a working tree, and know the quilt
mode, and know we are pushing or building.

There is one call to build_or_push_prep_modes near every call to
build_or_push_prep_early - after pushing/notpushing (which also means
after parseopts_late_defaults).

There is one nontrivial functional difference: Now we use the format
from the working tree, rather than from the .dsc.  But these ought to
be identical in any sane situation.  If they are not then the user has
done something very strange.  The quilt fixup machinery may go wrong,
but we should detect a problem later, during the dpkg-source -x check,
so nothing wrong will be pushed anywhere.

Otherwise no overall functional change other than to debug output.

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