ending simultaneous slurs from partcombine

[Keith OHara]

Bug Squad,
  Would you take a look at this?
Part-combining has always had warnings and awkwardness when combining parts 
slurs, tuplets, crescendos, etc.  Version 2.14 would warn about trying to start 
two slurs on the first beat, but probably since the fix for issue 1967, 
LilyPond starts two slurs and lets one continue too long.

\version "2.15.39"
\partcombine
  {e'2( f'4 g') g'2( f')}
  {c'2( f'4 g') e'2( d')}

% Workarounds:
% Prevent the part-combiner from switching states during the slur
\partcombine
  {\partcombineChords e'2( f'4 g') \partcombineAutomatic g'2( f')}
  {c'2( f'4 g') e'2( d')}

% Use phrasing slurs
\partcombine
{e'2\( f'4 g'\) g'2\( f'\)}
{c'2\( f'4 g'\) e'2\( d'\)}