将序列化中[to_string]改为[to_string_pretty]进行格式化，改善显示效果

src/models/tree.rs

@@ -41,7 +41,7 @@ impl Tree {
 
     pub fn save(&self) -> String {
         let s = store::Store::new();
-        let tree_data = serde_json::to_string(&self).unwrap();
+        let tree_data = serde_json::to_string_pretty(&self).unwrap();
         let hash = s.save(&tree_data);
         hash
     }