Allow cycles in DiffTrees

The variation diff with a maximal matching (#ifdef A and #ifdef B are
both matched) of the following diff contains a cycle.

```diff
-#ifdef B
 #ifdef A
+#ifdef B
Line
```

Author: ibbem

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

@@ -446,7 +446,7 @@ public boolean test(final DiffNode d) {
                 }
                 // We detected a cycle because we visited a node but did not determine its value yet!
                 // Thus, we are stuck in a recursion.
-                case VISITED -> false;
+                case VISITED -> true;
                 case ALL_PATHS_END_AT_ROOT -> true;
                 case NOT_ALL_PATHS_END_AT_ROOT -> false;
             };