fix: VariationNode::stealChildrenOf once again

We have to create a copy of the children list because it is cleared by
some implementations of removeAllChildren(). So effectively, no children
were added via addChildren.

Author: pmbittner

src/main/java/org/variantsync/diffdetective/variation/tree/VariationNode.java

@@ -282,7 +282,7 @@ public void drop() {
      * @see removeAllChildren
      */
     public void stealChildrenOf(final T other) {
-        List<T> children = other.getChildren();
+        List<T> children = new ArrayList<>(other.getChildren());
         other.removeAllChildren();
         addChildren(children);
     }