Slur collision detection and resolution works for sharp symbol but fails for flat symbol

[Jürgen Reuter]

Looks like slur collision detection and resolution (the slur is moved upwards) 
works fine for sharp accidentals, but is ignored for flat accidentals.


Lily-Code (here tested on current lilybin.com, but seems to be reproducible in 
all lily versions of the last couple of years, hence it's probably not a recent 
regression):


% LilyBin
\score{
    {
        \override Stem.direction = #UP
        \override Slur.direction = #UP
        a'( g'') a'( g'')
        a'( ges'') a'( gis'')
    }

    \layout{}
    \midi{}
}


Best wishes,

Jürgen