Merged noteheads getting filled with NoteColumn #'ignore-collision = ##t

[Nick Payne]

Following test code demonstrates the problem on my system. I obviously don't
need ignore-collision set for this particular example, but I do for the
score where I noticed the problem. Can I stop the merged noteheads from
getting filled in this situation?

\version "2.13.3"

\relative d {
        \key d \major
        \mergeDifferentlyHeadedOn
        << {
                d16 fis' a d fis a, d fis d, fis a d fis a, d fis |
                \override NoteColumn #'ignore-collision = ##t
                d,, fis' a d fis a, d fis d, fis a d fis a, d fis |
        }
        \\ {
                d,,2 d' |
                d, d' |
        }
        >>
}

Nick