code: public GitlabDiscussion getDiscussion(GitlabMergeRequest mergeRequest,
              int discussionId) throws IOException {
          String tailUrl = GitlabProject.URL + "/" + mergeRequest.getProjectId() +
                  GitlabMergeRequest.URL + "/" + mergeRequest.getIid() +
                  GitlabDiscussion.URL + "/" + discussionId;
          return retrieve().to(tailUrl, GitlabDiscussion.class);
      }
docs: Get a discussion by id from a merge request.
  <a href="https://docs.gitlab.com/ce/api/discussions.html#get-single-merge-request-discussion">https://docs.gitlab.com/ce/api/discussions.html#get-single-merge-request-discussion</a>
  
  @param mergeRequest to fetch the discussion from.
  @param discussionId The id of the discussion.
  
  @return The GitLab discussion identified by the given id.
  @throws IOException on a GitLab api call error