verbose message when prealloc write

src/main.rs

@@ -98,6 +98,7 @@ async fn write_graph(graph: Arc<data::HierarchicalINodeGraph>) -> eyre::Result<(
 		.wrap_err(eyre!("Failed to open file for mapping"))
 		.with_section(|| format!("{:?}", output_file).header("File was"))?;
 	    let mut file = file.into_std().await;
+        cfg_eprintln!(Verbose; cfg, "Opened file for prealloc-write");
 	    tokio::task::spawn_blocking(move || {
 		serial::write_sync_map(&mut file, graph.as_ref())
 	    }).await.wrap_err(eyre!("Prealloc panicked while dumping"))