Issue 3639: tuplet of a single chord gives a syntax error (issue 20320044)

[k-ohara5a5a]

I have been reading, slowly absorbing this and the patch for issue 3618,
and see no problems so far.

I see now that the challenge is not so much determining that \repeat...
is not a duration for
  \tuplet 6/4 \repeat tremolo 3 {g8 b}
but in collecting enough information to let ly:music? determine that
\repeat... is in fact music, should anyone write a function that has an
optional argument of type music (which I did write and which does now
work).

I think this repairs all the realistic cases, where  {\relative <c' e g
c>4 } is the limit of what I considered realistic.

https://codereview.appspot.com/20320044/