code: function updateManifest (archive, updates, cb) {
    return maybe(cb, async function () {
      var manifest
      try {
        manifest = await readManifest(archive)
      } catch (e) {
        manifest = {}
      }
      Object.assign(manifest, generateManifest(updates))
      return writeManifest(archive, manifest)
    })
  }
docs: helper to write updates to a manifest object