Regression: Completion heads mess up lyrics

[Arvid Grøtting]

> I'm not top-posting.

% \version "2.11.39"

% lyrics are misaligned when using completion heads.
% They should be in the same place below both staves.
% Bug exists in both 2.11.39 and 2.10.33
% This is a regression; it worked as it should in 2.8.8 and in 2.9.27.

mel = \relative c'' {
  c2 c1 c1 c2
}

lyr = \lyricmode {
  One -- two __ three -- four.
}

\score {
  \new Score <<
    \new Staff <<
      \new Voice = "normal" \mel
    >>
    \new Lyrics \lyricsto "normal" \lyr
    \new Staff <<
      \new Voice = "completion" \with {
        \remove "Note_heads_engraver"
        \remove "Forbid_line_break_engraver"
        \consists "Completion_heads_engraver"
      } \mel
    >>
    \new Lyrics \lyricsto "completion" \lyr
  >>
  \layout {}
}