Follow-up from "Incorporate Gitaly's CommitService.FindCommit RPC"

The following discussion from gitlab-ce!13094 should be addressed:

• @dzaporozhets commented on a discussion: (+3 comments)

  Renamed to rugged_diff_from_parent. If you don't mind, I'll open another MR to standardize the gitaly methods naming.

  @eReGeBe sure. Please create an issue first with gitlab-ce~3011693 label