Clean up co-linear tracks after finishing routing

This involved adding some extra infrastructure to be able
to handle the case where a track that is sitting in the
router's commit waiting to be added to the board actually
needs to be deleted instead.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/8419

common/commit.cpp

@@ -144,6 +144,21 @@ void eraseIf( Container& c, F&& f )
 }
 
 
+COMMIT& COMMIT::Unstage( EDA_ITEM* aItem )
+{
+    if( m_changedItems.find( aItem ) != m_changedItems.end() )
+    {
+        eraseIf( m_changes,
+                [aItem] ( const COMMIT_LINE& aEnt )
+                {
+                    return aEnt.m_item == aItem;
+                } );
+    }
+
+    return *this;
+}
+
+
 COMMIT& COMMIT::createModified( EDA_ITEM* aItem, EDA_ITEM* aCopy, int aExtraFlags )
 {
     EDA_ITEM* parent = parentObject( aItem );
@@ -215,3 +230,16 @@ CHANGE_TYPE COMMIT::convert( UNDO_REDO aType ) const
     }
 }
 
+
+std::vector<EDA_ITEM*> COMMIT::GetAddedItems() const
+{
+    std::vector<EDA_ITEM*> ret;
+
+    for( const COMMIT_LINE& change : m_changes )
+    {
+        if( change.m_type == CHT_ADD )
+            ret.emplace_back( change.m_item );
+    }
+
+    return ret;
+}