Resolve "Return how many commits the source branch is behind the target branch through the API"

What does this MR do?

Adds diverged_commits_count field to GET api/v4/projects/:project_id/merge_requests/:merge_request_iid

The field is optional and returned only if diverged_commits_count is true; the default value is false.

What are the relevant issue numbers?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

Closes #36534 (closed)