Fix determine-frets so that it preserves note order (issue4518045)

[Carl . D . Sorensen]

Reviewers: MikeSol,

Message:
In response to Mike's request, I've fixed the problem in determine-frets
that sometimes changed the order of the notes in the chord.

This keeps the glissando between the same notes in the TabStaff as in
the Staff.

I've made enough changes that I'd like to get a review of the changes.

Thanks,

Carl


Description:
Fix determine-frets so that it preserves note order

Please review this at http://codereview.appspot.com/4518045/

Affected files:
  M input/regression/tablature-harmonic.ly
  M scm/translation-functions.scm