Add diffs type to commit on graphql

What does this MR do and why?

Adds an endpoint under commits on GraphQL. This mr is intended to allow us to query the diff of a specific commit or list of commits of a mr.

Fixes: https://gitlab.com/gitlab-com/account-management/eastern-north-america/red-hat/red-hat-kernel-team/-/issues/116
Fixes: #366639

Screenshots or screen recordings

How to set up and validate locally

Example query:

query MyQuery($namespace: ID!, $iid: String!) {
  project(fullPath: $namespace) {
    mergeRequest(iid: $iid) {
      commits {
        nodes {
          diffs {
            bMode
            aMode
            oldPath
            newPath
            newFile
            diff
            deletedFile
          }
        }
      }
    }
  }
}

{
  "namespace": "jashkenas/Underscore",
  "iid": "1"
}

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.