Fix AppendGraph::capacity to actually use the underlying vec's capacity

src/models/append_graph.rs

@@ -91,7 +91,7 @@ impl GraphTopology for AppendGraph {
     }
 
     fn vertex_capacity(&self) -> usize {
-        self.vertices.len()
+        self.vertices.capacity()
     }
 
     fn vertex_map<T: Clone>(&self, default: T) -> VertexMap<Self::Vertex, T> {