Incorporate Gitaly's CommitService.FindCommit RPC

What does this MR do?

Incorporate Gitaly's CommitService.FindCommit RPC

Are there points in the code the reviewer needs to double check?

The refactors to Gitlab::Git::Commit.

Why was this MR needed?

gitaly#321 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes gitaly#402 (closed)