Add diff count to CommitStats RPC

Background

For commit rapid diffs we need to get a count of all the diff files to populate this badge on the commit page:

Currently to get the count we are querying Gitaly for all diffs. Separately for rapid diffs we are querying for the first 5 diffs. As per @patrickbajao's comment we could try to use git diff --numstat to return the number of changed files and add this to the CommitStats RPC.

$ git diff --numstat
5       1       app/services/ci/job_artifacts/create_service.rb
22      0       spec/services/ci/job_artifacts/create_service_spec.rb

Proposed solution

A new files property is being added to CommitStats RPC in gitaly!7393 (merged).

From gitlab-rails perspective, we need to:

1. Consume the new files property from RPC response
2. Use the Gitlab::Git::Commit#stats in commit page for rapid diffs.