Critical bug of midi output

[胡海鹏 - Hu Haipeng]

Hello,
  I found a problem of Midi output in 2.13.56. If you write a timpani roll like this:

\repeat tremolo 4 c32\mp\< \repeat tremolo 8 c c8\!f

and use unfold in midi  block, The output will sound weird. The second note is already \f. This problem is very obviously when I write some long notes with a long crescendo on sustained instruments.
  Please compare the two midi files in this zip file, and you'll know how ugly 2.13.56 renders it.
http://www.darajan2.com/douban/Haipeng/midi.zip
  If anyone need to observe the code, I'll send the files.

Regards
Haipeng