End the concurrent slurs created by partcombine; issue 2584 (issue 6272046)

[dak]

http://codereview.appspot.com/6272046/diff/5/input/regression/slur-multiple.ly
File input/regression/slur-multiple.ly (right):

http://codereview.appspot.com/6272046/diff/5/input/regression/slur-multiple.ly#newcode17
input/regression/slur-multiple.ly:17: \relative c'' {
If you want a different test case, maybe add it to the end?  Otherwise
one can't judge the effect of your changes on the existing regtest.

http://codereview.appspot.com/6272046/diff/5/lily/slur-engraver.cc
File lily/slur-engraver.cc (right):

http://codereview.appspot.com/6272046/diff/5/lily/slur-engraver.cc#newcode185
lily/slur-engraver.cc:185: ended = true;
Why is this change going to work with double slurs?  Those have multiple
start events and only one end event.

http://codereview.appspot.com/6272046/diff/5/lily/slur-engraver.cc#newcode212
lily/slur-engraver.cc:212: else
What difference do you expect from this change?  it is not clear to me.

http://codereview.appspot.com/6272046/