cross staff notes collide

[Evan Laforge]

I looked at the issue tracker but couldn't find this one.  I was going
to go ahead and file a bug directly, but then it told me to bring it
up here first, so here I am.  It might be related to 721 "Dynamics can
collide with notes when applied to cross-staff voices" if it's simply
that cross staff notes collide with everything :)

Example code:

\version "2.16.2"

\score {
<<
\new PianoStaff <<

\new Staff = "up" {
{
  f''8 e''8 e'' e''
} }

\new Staff = "down" {
{
  e'8 \change Staff = "up" g'8 g' g'
} }

>>
>>
}


When note heads cross to the upper staff, they should cause the upper
notes to flip stem directions to avoid collision.