Properly handle incremental bus connections

- Incremental extraction needs the newly created items.  The modified
  items are already set in the connection graph but the newly created
  items don't exist yet, so we need to add those explicitly
- Bus parents need to ensure that the bus children exist because we
  iterate on the updates from the top of the graph down

eeschema/connection_graph.cpp

@@ -233,13 +233,20 @@ void CONNECTION_SUBGRAPH::getAllConnectedItems( std::set<std::pair<SCH_SHEET_PAT
     CONNECTION_SUBGRAPH* sg = this;
 
     while( sg->m_absorbed_by )
+    {
+        wxASSERT( sg->m_graph == sg->m_absorbed_by->m_graph );
         sg = sg->m_absorbed_by;
+    }
 
     aSubgraphs.insert( sg );
     aSubgraphs.insert( sg->m_absorbed_subgraphs.begin(), sg->m_absorbed_subgraphs.end() );
 
     for( SCH_ITEM* item : sg->m_items )
+    {
+        item->Connection( &m_sheet )->Reset();
+        item->ConnectedItems( m_sheet ).clear();
         aItems.emplace( m_sheet, item );
+    }
 
     for( CONNECTION_SUBGRAPH* child_sg : sg->m_hier_children )
         child_sg->getAllConnectedItems( aItems, aSubgraphs );
@@ -604,8 +611,11 @@ void CONNECTION_GRAPH::Recalculate( const SCH_SHEET_LIST& aSheetList, bool aUnco
         for( SCH_ITEM* item : sheet.LastScreen()->Items() )
         {
             if( item->IsConnectable() && ( aUnconditional || item->IsConnectivityDirty() ) )
+            {
+                wxLogTrace( ConnTrace, wxT( "Adding item %s to connectivity graph update" ),
+                            item->GetTypeDesc() );
                 items.push_back( item );
-
+            }
             // Ensure the hierarchy info stored in SCREENS is built and up to date
             // (multi-unit symbols)
             if( item->Type() == SCH_SYMBOL_T )
@@ -663,11 +673,17 @@ std::set<std::pair<SCH_SHEET_PATH, SCH_ITEM*>> CONNECTION_GRAPH::ExtractAffected
     {
         // Find the primary subgraph on this sheet
         while( aSubgraph->m_absorbed_by )
+        {
+            wxASSERT( aSubgraph->m_graph == aSubgraph->m_absorbed_by->m_graph );
             aSubgraph = aSubgraph->m_absorbed_by;
+        }
 
         // Find the top most connected subgraph on all sheets
         while( aSubgraph->m_hier_parent )
+        {
+            wxASSERT( aSubgraph->m_graph == aSubgraph->m_hier_parent->m_graph );
             aSubgraph = aSubgraph->m_hier_parent;
+        }
 
         // Recurse through all subsheets to collect connected items
         aSubgraph->getAllConnectedItems( retvals, subgraphs );
@@ -2229,6 +2245,9 @@ void CONNECTION_GRAPH::propagateToNeighbors( CONNECTION_SUBGRAPH* aSubgraph, boo
                                     candidate->m_code, candidate->m_driver_connection->Name() );
 
                         candidate->m_hier_parent = aParent;
+                        aParent->m_hier_children.insert( candidate );
+
+                        wxASSERT( candidate->m_graph == aParent->m_graph );
 
                         search_list.push_back( candidate );
                         break;