dgit: split brain reorg: Always _needed in split brain quilt modss

dgit: split brain reorg: Always _needed in split brain quilt modss

The only differences are:

1. Call quiltify_splitbrain_needed earlier.  This is of no
consequence.

2. Call it even dpm mode when .gitignore was not edited.  This is
actually somewhat more correct.  In dpm mode we are very likely later
going to need split brain to keep the pseudomerge of the dgit view
off the maintainer branch.

And in fact that is what will happen because dopush fetches the quilt
view from the cache and that sets $split_brain unconditionally.  With
the current code structure dgit's quilt fixup imagines that it can do
a quilt fixup for dpm mode without split brain, when the .gitignore is
not edited.  But actually what is happening is that this is an empty
tree fixup, and the history inclusion *will* be done split brain, but
later.

So overall this change will have no actual overall effect.  But it is
a move towards helping untangle the quilt mode from split brain.

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