Jane found out that abc2midi output goes rather random sometimes. I've narrowed
this down to sections of music after a [1..[2 |] repeat has happened.
Work around by adding explicit |: for those sections, which doesn't (to my
mind) look so good printed but gets correct behaviour from abc2midi.
1. Only show tune markers when a tune change is in the middle of a line.
2. Only show repeat starts |: if the repeat start is in the middle of a line.
3. Final bar end mark must be a thick line.
The official rules for a repeat are that if there is no left repeat
marker |: then the repeat starts at the previous double bar.