Rename `Entities::RepoXxx` to `Entities::Xxx`

As noted in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9351#note_23749577, I don't see the point of having RepoCommit, RepoCommitStats, and RepoCommitDetail. We should drop the Repo prefix and call it a day. That would allow us to drop the hack from !9351 (merged).