dgit: split brain reorg: Move some split brain code further out.

dgit: split brain reorg: Move some split brain code further out.

We are going to want this in other quilt modes.  Logically, it should
occur in build_maybe_quilt_fixup.  Specifically, move:
 * the quilt cache check
 * the splitting of the dgit-view branch and setting $split_brain
out of quilt_fixup_multipatch.  quilt_fixup_multipatch now gets
$upstreamversion as a parameter.

Right now this logic in build_maybe_quilt_fixup is a bit flabby and
confusing.  We hope to tidy it up a bit later.

No functional change.

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