Additions and deletions not present in REST API for merge request diff stats
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### Summary
The [Merge Request Diff REST API](https://docs.gitlab.com/ee/api/merge_requests.html#list-merge-request-diffs) does not contain the count of lines added, deleted and changes in the respone but the [Merge Request GraphQL API](https://docs.gitlab.com/ee/api/graphql/reference/#mergerequest) does have a field [`diffStatsSummary`](https://docs.gitlab.com/ee/api/graphql/reference/#diffstatssummary) which shows total lines added, deleted and changed as part of the merge request
### Steps to reproduce
1. Make a GET request on the REST API `/projects/:id/merge_requests/:merge_request_iid/diffs` and observe that the individual changes are listed but a summary of the changes is not present
2. Use the GraphiQL Explorer and use the below query and observe that the diff summary is listed
```
{
project(fullPath: "gitlab-org/gitlab") {
mergeRequest(iid: "154530") {
diffStatsSummary {
additions
deletions
changes
}
}
}
}
```
### Example Project
https://gitlab.com/gitlab-org/gitlab
### What is the current *bug* behavior?
The summary of diff is not present in REST API
### What is the expected *correct* behavior?
The summary of diff should also be present in REST API
### Relevant logs and/or screenshots
issue