message: should not fail here on non-string id
  our validation layer will catch it further down, with a nicer error message
diff: @@ -116,7 +116,7 @@ module.exports = class Writer {
  authorEmail: user ? user.attributes.email : 'anon@example.com',
  committerName: this.myName,
  committerEmail: this.myEmail,
  - message: `${operation} ${type} ${id.slice(12)}`
  + message: `${operation} ${type} ${String(id).slice(12)}`
  };
  }
  