git-debrebase: merge: Detect octopi later

If git-debrebase or dgit were to make an octopus merge, we should
recognise that as an unknown kind of gdr/dgit merge, not as a general
octopus merge.  This code motion leaves a spot where such a new kind
of merge could be classified.

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