Shapenote shapes in minor

[Robert Stoddard]

> I'm not top posting.

A longstanding bug remains unaddressed in 2.13.50.  The following tiny example 
shows that the shapes are wrong when the key is in minor; the workaround, as 
shown, is to set the \key to the relative major.  

\version "2.13.50"
myLyrics = \lyricmode { la so la mi fa so la la }
myMusic = \relative g' { a4 g a b | c d e a, }
\new Staff { 
        \sacredHarpHeads
        \key a \minor
        \myMusic
        \bar "."
        \key c \major
        \myMusic
}
\addlyrics { \myLyrics \myLyrics }