fix: VariationNode::stealChildrenOf

Previously, this method always crashed because addChildren assumes the
given children to have no parent. Yet, the children always had 'other'
as parent because we only removed them afterwards.

Author: pmbittner

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

@@ -282,8 +282,9 @@ public void drop() {
      * @see removeAllChildren
      */
     public void stealChildrenOf(final T other) {
-        addChildren(other.getChildren());
+        List<T> children = other.getChildren();
         other.removeAllChildren();
+        addChildren(children);
     }
 
     /**