Add MigrateRootNodes option with 'false' default to preserver backward compatibility

Author: satano

src/MigrationTools.Clients.TfsObjectModel/Tools/TfsNodeStructureTool.cs

@@ -466,8 +466,11 @@ private void ProcessCommonStructure(string treeTypeSource, string localizedTreeT
 
             _pathToKnownNodeMap[structureParent.Path] = structureParent;
 
-            XmlElement mainNode = sourceTree.ChildNodes.OfType<XmlElement>().First();
-            CreateNewRootNode(mainNode, nodeStructureType);
+            if (Options.MigrateRootNodes)
+            {
+                XmlElement mainNode = sourceTree.ChildNodes.OfType<XmlElement>().First();
+                CreateNewRootNode(mainNode, nodeStructureType);
+            }
             if (sourceTree.ChildNodes[0].HasChildNodes)
             {
                 // The XPath would look like this: /Nodes/Node[Name=Area]/Children/...

src/MigrationTools.Clients.TfsObjectModel/Tools/TfsNodeStructureToolOptions.cs

@@ -30,6 +30,15 @@ public sealed class TfsNodeStructureToolOptions : ToolOptions, ITfsNodeStructure
         /// </summary>
         public bool ShouldCreateMissingRevisionPaths { get; set; }
         public bool ReplicateAllExistingNodes { get; set; }
+
+        /// <summary>
+        /// By default, only child nodes in node structures are migrated. Turning this on migrates even the root node
+        /// (default area/iteration). This is useful, when you do not have any child areas ion source, but want to migrate
+        /// default area in source to some sub-area in target project. You have to set proper mapping for this to work.
+        /// If after mapping the target node is still just root node (so it is not migrated under some child node),
+        /// nothing happens – so migrating root nodes works only if the are remapped to some child node in target project.
+        /// </summary>
+        public bool MigrateRootNodes { get; set; } = false;
     }
 
     public class NodeOptions