one-line percussion staff: bar rest on and not under!

[Antti Kaihola]

Here's my single staff line example again. There's an R1 there, and the
bar rest symbol floats above the staff line instead of hanging from it
as it should.

\context StaffGroup <<
    \new DrumStaff \with { \override StaffSymbol #'line-count = #1 }
    \drummode { mar2 mar | R1 | \repeat volta 2 { mar2 mar | } }
    \new Staff \relative { c4 d e f | g a b c | \repeat volta 2 { b a g f } }
>>