key-performer.cc: use Pitch::negated for readability (issue 333500043 by address@hidden)

[nine . fierce . ballads]

https://codereview.appspot.com/333500043/diff/1/lily/key-performer.cc
File lily/key-performer.cc (right):

https://codereview.appspot.com/333500043/diff/1/lily/key-performer.cc#newcode71
lily/key-performer.cc:71: key_do.negated ().smobbed_copy ());
LGTM, but ...

I also have a hard time understanding the existing code in this
function.

Isn't it sufficient to check whether the interval between pitch-alist[0]
and pitch-alist[2] is a minor third, i.e. 3 half-steps?  I don't see why
the key signature would need to be transposed to C for that.  Maybe
there's a music-theoretical concern I'm overlooking.

https://codereview.appspot.com/333500043/