message: Gracefully handle GitHub plugin git exec failures
diff: @@ -129,7 +129,7 @@ class Repository {
  const out = <ShellJS.ExecOutputReturnValue> ShellJS.exec('git rev-parse --show-toplevel', {silent: true});
  ShellJS.popd();
  - if (out.code !== 0) {
  + if (!out || out.code !== 0) {
  return null;
  }
  return new Repository(BasePath.normalize(out.stdout.replace('\n', '')), gitRevision);
  