Provide support in update MR api to update attribute 'merge_commit_message_with_description'

Below API does not expose attribute 'merge_commit_message_with_description' or similar fields which can be appeared in GIT LOG. https://docs.gitlab.com/ee/api/merge_requests.html#update-mr

This helps us in Git level automation instead querying directly to Gitlab server to fetch the description from Gitlab MR.