Add project.repository.commits query to GraphQL

What does this MR do and why?

Add project.repository.commits query to GraphQL

This attaches the Repositories::CommitsResolver to the RepositoryType object for GraphQL. This allows us to query for commits from a repository.

Currently experimental while we monior performance and develop the feature.

Changelog: added

References

CommitList GraphQL query (#541920 - closed)

How to set up and validate locally

In the GDK go to the /~/graphql-explorer and use the query

{
  project(fullPath: "gitlab-org/gitlab-test") {
    id
    repository {
      commits(
        ref: "master"
        query: "Merged"
        author: "Stan"
        committedBefore: "2020-01-01"
        committedAfter: "2015-01-01"
        first: 4
      ) {
        pageInfo {
          hasNextPage
          startCursor
          endCursor
        }
        nodes {
          sha
        }
      }
    }
  }
}

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