hash: return file contents to avoid multiple syscalls and allocations

lib/hash.js

@@ -9,9 +9,9 @@ import { readFile } from 'fs-promise';
  */
 
 export default async function hashes (files) {
-  const entries = await Promise.all(files.map(async (file) => {
-    const data = await readFile(file);
-    return [hash(data), file];
+  const entries = await Promise.all(files.map(async (name) => {
+    const data = await readFile(name);
+    return [hash(data), { name, data }];
   }));
   return new Map(entries);
 }