feat: add merge function

Able to deal with:
1.  merge a subtree

packages/file-graph/utils.js

@@ -1,5 +1,8 @@
 export function dirname(path) {
   const parts = path.split('/')
+  if (parts.length <= 2) {
+    return '/'
+  }
   parts.pop()
   return parts.join('/')
 }