Implement CommitDelta RPC

~Conversation: #566 (closed)

This MR implements CommitDelta RPC under the Diff service. It's been made a separate RPC so we can control how much deltas can go in a message, as deltas should be way smaller than actual diffs (due to the absence of chunks). GitLab-CE calls to Commit#raw_diffs(deltas_only: true) should be handled by this RPC.

Part of #178 (closed).