Gitaly parent ids array

Target branch is not master, but 10-5-stable. Equivalent changes for master: gitlab-org/gitlab-ce!17237

On a repeated field the type is Google::Protobuf::RepeatedField. However, an Array is preferable as the interface is predictable and its expected at least not to be a Google::Protobuf type either way in the main app, that is, not in lib/gitlab/git. All this would've been prevented when not using a repeated field but just streaming the commits back, and in my opinion this is the way it should be refactored after the Gitaly v1.0 release when designing the interface for 2.0.