It doesn't work. git_repository_open_ext throw System.ArgumentNullException

```csharp
        [DllImport(libgit2)]
        internal static extern int git_repository_open_ext(
            ref NullRepositorySafeHandle repository,
            [MarshalAs(UnmanagedType.CustomMarshaler, MarshalTypeRef = typeof(FilePathMarshaler))] FilePath path,
            int flags,
            [MarshalAs(UnmanagedType.CustomMarshaler, MarshalTypeRef = typeof(FilePathMarshaler))] FilePath ceilingDirs);

NullRepositorySafeHandle safeHandle = null;
Proxy.git_repository_open_ext(ref safeHandle, path, 1, null);
```