RevisionGraph: Add "Copy ref names" 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>
RevisionGraph: Drop unused code

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