RevisionGraph: Add Repository Browser context menu item

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
RevisionGraph: Add Switch/Checkout context menu item

Signed-off-by: Sup Yut Sum <ch3cooli@gmail.com>
RevisionGraph: Add Merge context menu item

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