Split brain: Start work on determining split brain necessity

`Split brain' means the situation where `dgit push' makes private
commits, to push to the dgit repos server, which are not left on the
user's actual branch.

Check whether the quilt mode and tree differences, taken together,
mean that we need split brain.

If we do need split brain, use gbp pq to apply the patch series.

.gitignore changes are mishandled, still.  And we don't actually
follow through much yet.

Also, fix a case where quilt mode `apply' hadn't been changed to
`unapplied'.