Parent reference in selector with @extends produce wrong output

[xzyfer]

Originally reported on sass/sassc#137

.mango {
  color: red;
}

.something {
  &__else {
    @extend .mango;
    color: blue;
  }
}

Ruby Sass

.mango, .something__else {
  color: red; }

.something__else {
  color: blue; }

LibSass 3.2.5

.mango, .something__else {
  color: red; }

.something__else {
  color: blue; }

LibSass master

mango, __else.something {
  color: red; }

.something__else {
  color: blue; }

Spec added sass/sass-spec#478