partcombiner should not set "solo" when a rest is printed in the other voice

[Urs Liska]

Hi all,

consider the following \partcombine situation:
- voices are merged to a given point
- then voices start with different rests,
  followed by notes in one voice

\version "2.19.16"

one = \relative c'' {
  c2 c
  \voiceOne % this doesn't affect the "solo" section at all
  r8
    %\voiceOne % uncommenting this will make the voice look right
    c b a g f e d
  c1
}

two = \relative c' {
  c2 c
  R1*2
}

\score {
  \new Staff \partcombine \one \two
}

Here part \two prints a full measure rest. Part \one would *currently* 
print nothing and will print a crotchet rest when issue 4205 has been 
pushed (the attached image is created with that patch applied).
When the crotchets in \one start, the partcombiner sets the upper voice 
to "solo" because \two isn't playing at that moment. But it should be 
set to the upper voice instead:

- We don't want to read "Solo" when there's *something* (i.e. the rest) 
printed in the other voice
- the line should be \voiceOne and not \oneVoice because there *is* the 
rest in \voiceTwo.

Please add this to the tracker (after a second pair of eyes has gone 
over it.

TIA
Urs

partcombine-merge-rests.png
Description: PNG image