Bring the GitRemoteCallbacks structure in sync with libgit2

[arrowd]

The marshaling was messed up due to missing extra field.

Fixes #2161

[bording]

@arrowd I don't think you were looking at the right version of the libgit2 code when doing your investigations.

LibGit2Sharp is currently using v1.8.4, and that version of the git_remote_callbacks struct doesn't have the field you've added: https://github.com/libgit2/libgit2/blob/v1.8.4/include/git2/remote.h#L557-L658

[arrowd]

The mentioned issue was talking exactly about using non-bundled libgit2 version.

I realize that this PR can't be merged as of now, but it will come in handy if libgit2sharp would bump its bundled libgit2.

[bording]

The mentioned issue was talking exactly about using non-bundled libgit2 version.

A non-bundled version still needs to be v1.8.4 to be compatible with LibGit2Sharp.

I'm going to go ahead and close this for now then.