alternatives not taken into account in automatic accidentals

[Frédéric Bron]

This is a bug: automatic accidentals do not take into account
alternatives after volta repeat:

{
  #(set-accidental-style 'modern)
  \time 2/4
  \repeat volta 2 { cis'4 dis' }
  \alternative { { c' d'8 eis'8 } { d'4 c'8 e' } }
}

in the second alternative: we should have:
- natural d, natural c, no sign e
instead of
- no sign d, no sign c, natural e

Frédéric