notes that cross barlines display wrong length under \scaleDurations

[Graham King]

> I'm not top posting.

In trying to create a modern edition of a piece of renaissance polyphony,
I've found a problem in the interaction between \scaleDurations and the
Completion_heads_engraver.  I've left 3 lines of comments at the top of the
code to give some idea why I want to do this crazy thing :)

The problem is that any note crossing a barline is displayed with twice its
proper duration.

Adjust the comments at lines 10 and 13 below, to trigger the bug or not, and
observe the length of the two notes that cross barlines.

\version "2.16.0"

tenorMusic = \relative c' {
    % Note: Tenor is in tempus perfectum while other parts are in cut-C, 
    % so (in MS) tenor 
    % perfect brevis is worth 3 breves (sic) of the other parts.
    \scaleDurations 2/1 {
        \time 3/2     % unaffected by \scaleDurations
        \set Timing.measureLength = #(ly:make-moment 6 2)
        % r2   % uncomment this line, ...
        r2 r2  % (by the way, r2*2 is another bug)
        e1. b'4 a g1 g4 f2 f4 e2 e d c
        f      % ... and comment out this line, to fix note values.
        \bar "||"
    }
}

\score {
    \context Staff = tenor <<
        \context Voice = tenor \with {
            % note values are correct if we leave 
            % Note_heads_engraver active...
            \remove "Note_heads_engraver"
            \remove "Rest_engraver"
            % ... and don't use Completion_heads_engraver
            \consists "Completion_heads_engraver"
            \consists "Completion_rest_engraver"
        }
        <<
                \tenorMusic 
        >> 
    >>
    \layout {
    }
}