dgit: quilt fixup: Move split brain branch creation out a bit

dgit: quilt fixup: Move split brain branch creation out a bit

Move the playtree git branch, and the setting $split_brain from
quiltify_splitbrain (inside quilt_fixup_multipatch, conditional on
quiltmode_splitbrain) to quilt_fixup_multipatch.

This is still not quite right - it needs to be further out, because it
should affect quilt_fixup_singlepatch too.  Right now singlepatch DTWT
in split brain mode.

No functional change for now, anyway.

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