Add GoOdbBackend interface as a libgit2 Odb backend

Signed-off-by: Tibor Vass <teabee89@gmail.com>
Add helper functions handleError and gobool

Signed-off-by: Tibor Vass <teabee89@gmail.com>
Add GoRefdbBackend interface as a libgit2 Refdb backend

Signed-off-by: Tibor Vass <teabee89@gmail.com>