use functionExplanation for VDiff/Vtree::toString

Author: pmbittner

src/main/java/org/variantsync/diffdetective/variation/diff/VariationDiff.java

@@ -558,7 +558,7 @@ public boolean isSameAsIgnoringLineNumbers(VariationDiff<L> b) {
 
     @Override
     public String toString() {
-        return "VariationDiff of " + source;
+        return functionExplanation();
     }
 
     public VariationDiff<L> deepCopy() {

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

@@ -265,6 +265,6 @@ public String getSourceExplanation() {
 
     @Override
     public String toString() {
-        return "variation tree from " + source;
+        return functionExplanation();
     }
 }