Breathing sign doesn't avoid flag on 8th notes

[Nick Payne]

Happening on the top voice in single staff polyphony, hence the use of 
\stemUp to simulate that in the minimal example.

\version "2.13.7"0

\relative c'' {
   \time 5/8 \stemUp
   e2 e8 \breathe
}

Used \once \override BreathingSign #'extra-offset = #'(1 . 1.5)  to 
avoid the collision.

Nick