stem-shorten can't be reverted

[Werner LEMBERG]

In the example below, reverting the `stem-shorten' property fails.


    Werner

======================================================================

rechts = \notes \relative c'' {
  < a \\ e >
  < a \\
\property Voice.Stem \set #'stem-shorten = #'(0 0)
    e
\property Voice.Stem \revert #'stem-shorten
      >
  < a \\ e >
}

\score {
  \context Voice {
    \rechts
  }

  \paper {
    linewidth = -1
  }
}