More readable \displayMusic output. (issue 82420045)

[dak]

https://codereview.appspot.com/82420045/diff/1/scm/music-functions.scm
File scm/music-functions.scm (right):

https://codereview.appspot.com/82420045/diff/1/scm/music-functions.scm#newcode181
scm/music-functions.scm:181: (list (ly:moment-main obj))
Two-argument form is not really ambiguous: if second argument is
positive, it is a main moment denominator, otherwise a grace.

Since obj is a constant, that can always be verified.  In the very
unlikely case of a programmatically produced positive grace time, one
can still revert to the four-argument form.

https://codereview.appspot.com/82420045/diff/1/scm/music-functions.scm#newcode190
scm/music-functions.scm:190: ,@(if-needed (ly:duration-scale obj) 1)))
This would appear to turn
(ly:make-duration 2 0 2) into (ly:make-duration 2 2).

https://codereview.appspot.com/82420045/