Stems in TabStaff are not removed when manual beaming is used

[Federico Bruni]

As you can see in the attached image, the pull-off sign (slur in 
TabStaff) is moved up when using manual beaming.

It looks like Stems in default TabStaff are just made transparent when 
using manual beams. Maybe the stem stencil should be set to false also 
in this case.

The problem occurs on 2.14.2, 2.16.0 and latest build from git master.

\version "2.17.5"

first =  \relative c' {
  %% Stems in TabStaff are not removed when manual beaming is used
  e8 b g g' fis( e) d b % automatic beaming
  e8[ b g] g'[ fis( e)] d b % manual beaming

  %% behind the scenes...
  \tabFullNotation
  e8 b g g' fis( e) d b % automatic beaming
  e8[ b g] g'[ fis( e)] d b % manual beaming
}

second = \relative c {
  s1
}

\score {
  \new StaffGroup <<
    \new Staff <<
      \context Voice = "first voice" { \clef "G_8" \voiceOne  \first }
      \context Voice = "second voice" { \clef "G_8" \voiceTwo  \second }
    >>
    \new TabStaff <<
      %\tabFullNotation
      \context TabVoice = "tab first voice" { \clef "moderntab" 
\voiceOne \first }
      \context TabVoice = "tab second voice" { \clef "moderntab" 
\voiceTwo \second }
    >>
  >>
}

manual-beaming-and-stem-in-tabstaff.png
Description: PNG image