add referenced merge_requests endpoint

What does this MR do?

we have a process, that resets various repos on local-dev instanced based on the MR's related to an issue number. QA/dev - just enters the issue-id - via gitlab api we fetch the related mr's and reset the respective repos to that mr's branch.

doing it right now via some ugly hack:

related_mr_url = "http://gitlab.krone.at/KRN/MGMT/issues/#{issue_id}/referenced_merge_requests"
  #puts "Fetching url: #{related_mr_url}"
  out = RestClient::Request.execute( { :method => :get, :url => related_mr_url, :headers => {"PRIVATE-TOKEN" => ENV['KRN_GITLAB_TOKEN']} })

jso = JSON.parse out
  if jso["html"]
    mrs = jso["html"].scan(/href="\/(.*?)\/merge_requests\/(.*)"[^\>]*\>([^\<]+)\</)
    mrs.each do | mr |
      mrequest = client.merge_request(mr[0], mr[1])
    end
  end

this does not only feel ugly, buts a real bad thing.

this MR add's a new endpoint:

/api/v4/projects/:id/issues/:iid/referenced_merge_requests

just returning the merge requests:

[
  {
    "id": 29,
    "iid": 4,
    "project_id": 6,
    "title": "Provident eius eos blanditiis consequatur neque odit.",
    "description": "Ut consequatur ipsa aspernatur quisquam voluptatum fugit. Qui harum corporis quo fuga ut incidunt veritatis. Autem necessitatibus et harum occaecati nihil ea.\r\n\r\ntwitter/flight#8",
    "state": "opened",
    "created_at": "2018-09-18T14:36:15.510Z",
    "updated_at": "2018-09-19T07:45:13.089Z",
    "target_branch": "v2.x",
    "source_branch": "so_long_jquery",
    "upvotes": 0,
    "downvotes": 0,
    "author": {
      "id": 14,
      "name": "Verna Hills",
      "username": "lawanda_reinger",
      "state": "active",
      "avatar_url": "https://www.gravatar.com/avatar/de68a91aeab1cff563795fb98a0c2cc0?s=80&d=identicon",
      "web_url": "http://localhost:4001/lawanda_reinger"
    },
    "assignee": {
      "id": 19,
      "name": "Jody Baumbach",
      "username": "felipa.kuvalis",
      "state": "active",
      "avatar_url": "https://www.gravatar.com/avatar/6541fc75fc4e87e203529bd275fafd07?s=80&d=identicon",
      "web_url": "http://localhost:4001/felipa.kuvalis"
    },
    "source_project_id": 6,
    "target_project_id": 6,
    "labels": [],
    "work_in_progress": false,
    "milestone": {
      "id": 27,
      "iid": 2,
      "project_id": 6,
      "title": "v1.0",
      "description": "Et tenetur voluptatem minima doloribus vero dignissimos vitae.",
      "state": "active",
      "created_at": "2018-09-18T14:35:44.353Z",
      "updated_at": "2018-09-18T14:35:44.353Z",
      "due_date": null,
      "start_date": null,
      "web_url": "http://localhost:4001/twitter/flight/milestones/2"
    },
    "merge_when_pipeline_succeeds": false,
    "merge_status": "cannot_be_merged",
    "sha": "3b7b528e9353295c1c125dad281ac5b5deae5f12",
    "merge_commit_sha": null,
    "user_notes_count": 9,
    "discussion_locked": null,
    "should_remove_source_branch": null,
    "force_remove_source_branch": false,
    "web_url": "http://localhost:4001/twitter/flight/merge_requests/4",
    "time_stats": {
      "time_estimate": 0,
      "total_time_spent": 0,
      "human_time_estimate": null,
      "human_total_time_spent": null
    },
    "squash": false
  },
  {
    "id": 26,
    "iid": 1,
    "project_id": 6,
    "title": "Facere eum necessitatibus sequi aut veniam consectetur aperiam quidem laudantium nemo.",
    "description": "Itaque quibusdam explicabo ut autem. Sit dicta nihil maiores est ut nemo rem veritatis. Qui rem non nihil quod culpa.\r\n\r\n\r\ntwitter/flight#8",
    "state": "opened",
    "created_at": "2018-09-18T14:36:14.643Z",
    "updated_at": "2018-09-19T07:45:35.226Z",
    "target_branch": "alunny/publish_lib",
    "source_branch": "remove-from",
    "upvotes": 0,
    "downvotes": 0,
    "author": {
      "id": 14,
      "name": "Verna Hills",
      "username": "lawanda_reinger",
      "state": "active",
      "avatar_url": "https://www.gravatar.com/avatar/de68a91aeab1cff563795fb98a0c2cc0?s=80&d=identicon",
      "web_url": "http://localhost:4001/lawanda_reinger"
    },
    "assignee": {
      "id": 1,
      "name": "Administrator",
      "username": "root",
      "state": "active",
      "avatar_url": "https://www.gravatar.com/avatar/e64c7d89f26bd1972efa854d13d7dd61?s=80&d=identicon",
      "web_url": "http://localhost:4001/root"
    },
    "source_project_id": 6,
    "target_project_id": 6,
    "labels": [],
    "work_in_progress": false,
    "milestone": {
      "id": 30,
      "iid": 5,
      "project_id": 6,
      "title": "v4.0",
      "description": "Qui voluptatum omnis praesentium ullam nesciunt.",
      "state": "active",
      "created_at": "2018-09-18T14:35:44.432Z",
      "updated_at": "2018-09-18T14:35:44.432Z",
      "due_date": null,
      "start_date": null,
      "web_url": "http://localhost:4001/twitter/flight/milestones/5"
    },
    "merge_when_pipeline_succeeds": false,
    "merge_status": "cannot_be_merged",
    "sha": "daa75b9b17918f51f43866ff533987fda71375ea",
    "merge_commit_sha": null,
    "user_notes_count": 9,
    "discussion_locked": null,
    "should_remove_source_branch": null,
    "force_remove_source_branch": false,
    "web_url": "http://localhost:4001/twitter/flight/merge_requests/1",
    "time_stats": {
      "time_estimate": 0,
      "total_time_spent": 0,
      "human_time_estimate": null,
      "human_total_time_spent": null
    },
    "squash": false
  }
]

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#39157

/cc @smcgivern

Does this MR meet the acceptance criteria?

if the MR itself is OK, i am going to add tests/docs and stuff, also going to add it to narkoz/gitlab gem