code: function (cb) {
      exec('git log -n1 --format=format:\'%h\'', function (err, stdout) {
        if (err) {
          cb(err);
        }
        else {
          var commitId = stdout.replace(/'/g, '');
          cb(null, commitId);
        }
      });
    }
docs: get the latest commit ID as an 8-character string; if not a git repo, returns '' and logs an error; receiver is a function with the signature receiver(err, result), where err is null if no error occurred and result is the commit ID