Sorted tarball contents

This will make `npm pack` output the tarball contents
in an alphabetically and natural sort order.

The current implementation uses `localeCompare` with
the following options:

1. **sensitivity**: `base`, to correctly handle non-ascii letters
  using their base ascii, so that `á`,`â`, `ä`, `ã`, `å` are
  all handled like `a`

1. **numbers**: `true`, to handle numeric filenames as natural
  ascending order so that, for instance, file `10.txt` appears after `9.txt`

---

Convo from npm/rfcs#96

cc @ruyadorno @isaacs

test/index.js

@@ -180,3 +180,60 @@ t.test('packs from registry spec', async t => {
     await rimraf(testDir)
   })
 })
+
+t.test('files packed are alphabetically sorted', async (t) => {
+  const testDir = t.testdir({
+    index: 'index',
+    dist: {
+      'foo.js': 'foo',
+      'foo.ts': 'foo',
+      bar: 'bar',
+      baz: 'baz'
+    },
+    numbers: {
+      1: '1',
+      11: '1',
+      '01': '0',
+      20: '2',
+      2: '2',
+      10: '1',
+      0: '0'
+    },
+    'package.json': JSON.stringify({
+      name: 'pkg'
+    }),
+    lib: {
+      'index.js': 'lib/index',
+      'foo.js': 'lib/foo'
+    },
+    'README.md': 'readme-1',
+    'readme.txt': 'readme-2'
+  })
+
+  const target = `${testDir}/my-cool-pkg-1.0.0.tgz`
+  const tarContents = await pack(testDir, { target })
+  const tarFilePaths = tarContents.files.map(file => file.path)
+  const expectedOrder = [
+    'README.md',
+    'dist/bar',
+    'dist/baz',
+    'dist/foo.js',
+    'dist/foo.ts',
+    'index',
+    'lib/foo.js',
+    'lib/index.js',
+    'numbers/0',
+    'numbers/01',
+    'numbers/1',
+    'numbers/2',
+    'numbers/10',
+    'numbers/11',
+    'numbers/20',
+    'package.json',
+    'readme.txt'
+  ]
+
+  t.deepEqual(tarFilePaths, expectedOrder,
+    'files packed matches order expectations'
+  )
+})

utils/tar.js

@@ -86,6 +86,24 @@ async function getContents (manifest, target) {
     })
   ])
 
+  const comparator = (a, b) => {
+    return a.path.localeCompare(b.path, undefined, {
+      sensitivity: 'case',
+      numeric: true
+    })
+  }
+
+  const isUpper = (str) => {
+    const ch = str.charAt(0)
+    return ch >= 'A' && ch <= 'Z'
+  }
+
+  const uppers = files.filter(file => isUpper(file.path))
+  const others = files.filter(file => !isUpper(file.path))
+
+  uppers.sort(comparator)
+  others.sort(comparator)
+
   const shasum = integrity.sha1[0].hexDigest()
   return {
     id: manifest._id || `${manifest.name}@${manifest.version}`,
@@ -96,7 +114,7 @@ async function getContents (manifest, target) {
     shasum,
     integrity: ssri.parse(integrity.sha512[0]),
     filename: `${manifest.name}-${manifest.version}.tgz`,
-    files,
+    files: uppers.concat(others),
     entryCount: totalEntries,
     bundled: Array.from(bundled)
   }