message: ValueDiff::union(): Check that the diffs compose
diff: @@ -87,8 +87,8 @@ impl<T: PartialEq> ValueDiff<T> {
  (ValueDiff::Replace(a, b), ValueDiff::None) => {
  *self = ValueDiff::Replace(a, b);
  }
  - (ValueDiff::Replace(a, _b), ValueDiff::Replace(_c, d)) => {
  - //assert!(b == c); // FIXME
  + (ValueDiff::Replace(a, b), ValueDiff::Replace(c, d)) => {
  + assert!(b == c);
  if a == d {
  *self = ValueDiff::None;
  } else {
  