D5415: rust: changed Graph.parents to return [Revision; 2]

[Yuya Nishihara]

Queued the first two, thanks.

>   This is a nice improvement. It still seems odd to me to return a list of two parents when there could be 0, 1 or 2 parents. I guess this is for performance reasons?

It's a direct mapping from the revlog storage. I think `[Revision; 2]`
is good enough at this layer as I see the Graph trait is actually an
abstraction for storage-level objects.