Fixed issue #1750: Repository Browser fails for empty repository

Repository Browser should not say error if browsing orphan HEAD, just leave it empty like what Log Dialog does

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
Repository Browser: Do not show revert, copy path, copy hash menu items if no item selected

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>