Part combiner junks absolute dynamic

[Dan Eble]

> I'm not top posting.
% The part combiner keeps voices separate for crescendi spanning
% different intervals of time, but not for differing absolute
% dynamics.  For consistency, I think it should keep the voices
% separate for differing absolute dynamics.
\version "2.13.0"

soprano = \relative c'' { d4\mf d\< d d\! | d\f d\< d\! d |  }
alto = \relative c'' { g4\mf g\< g g\! | g4\mf g\< g g\! |  }

\score {
  \new Staff <<
    \partcombine \soprano \alto
  >>
}