Add GFM autocompletion for MergeRequests

Requested
[here](http://feedback.gitlab.com/forums/176466-general/suggestions/5507440-auto-complete-merge-requests-in-gitlab-markdown).

app/assets/javascripts/gfm_auto_complete.js.coffee

@@ -13,6 +13,7 @@ GitLab.GfmAutoComplete =
   Members:
     template: '<li data-value="${username}">${username} <small>${name}</small></li>'
 
+  # Issues and MergeRequests
   Issues:
     template: '<li data-value="${id}"><small>${id}</small> ${title} </li>'
 
@@ -46,11 +47,22 @@ GitLab.GfmAutoComplete =
         before_save: (issues) ->
           $.map issues, (i) -> id: i.iid, title: sanitize(i.title), search: "#{i.iid} #{i.title}"
 
+    input.atwho
+      at: '!'
+      alias: 'mergerequests'
+      search_key: 'search'
+      tpl: @Issues.template
+      callbacks:
+        before_save: (merges) ->
+          $.map merges, (m) -> id: m.iid, title: sanitize(m.title), search: "#{m.iid} #{m.title}"
+
     input.one "focus", =>
       $.getJSON(@dataSource).done (data) ->
         # load members
         input.atwho 'load', "@", data.members
         # load issues
         input.atwho 'load', "issues", data.issues
+        # load merge requests
+        input.atwho 'load', "mergerequests", data.mergerequests
         # load emojis
         input.atwho 'load', ":", data.emojis

app/controllers/projects_controller.rb

@@ -108,6 +108,7 @@ class ProjectsController < ApplicationController
     @suggestions = {
       emojis: Emoji.names,
       issues: @project.issues.select([:iid, :title, :description]),
+      mergerequests: @project.merge_requests.select([:iid, :title, :description]),
       members: @project.team.members.sort_by(&:username).map { |user| { username: user.username, name: user.name } }
     }