Fix name of function in NewReferenceNameIterator's comment

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

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

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

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