message: add tooltip message
  to indicate why the checkout is grayed out
diff: @@ -304,6 +304,8 @@ FileContextMenu::FileContextMenu(RepoView *view, const QStringList &files,
  foreach (const QString &file, files) {
  if (commit.tree().id(file) == repo.workdirId(file)) {
  checkout->setEnabled(false);
  + checkout->setToolTip(
  + tr("The file is already in the current working directory"));
  break;
  }
  }
  