Fetch merge request ref if it is missing when visit MR page

This will fix merge problem for merge requests between forks created
before 8.0
Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/controllers/projects/merge_requests_controller.rb

@@ -7,6 +7,7 @@ class Projects::MergeRequestsController < Projects::ApplicationController
   before_action :closes_issues, only: [:edit, :update, :show, :diffs, :commits]
   before_action :validates_merge_request, only: [:show, :diffs, :commits]
   before_action :define_show_vars, only: [:show, :diffs, :commits]
+  before_action :ensure_ref_fetched, only: [:show, :commits, :diffs]
 
   # Allow read any merge_request
   before_action :authorize_read_merge_request!
@@ -277,4 +278,10 @@ class Projects::MergeRequestsController < Projects::ApplicationController
       :state_event, :description, :task_num, label_ids: []
     )
   end
+
+  # Make sure merge requests created before 8.0
+  # have head file in refs/merge-requests/
+  def ensure_ref_fetched
+    @merge_request.ensure_ref_fetched
+  end
 end

app/models/merge_request.rb

@@ -433,10 +433,22 @@ class MergeRequest < ActiveRecord::Base
     target_project.repository.fetch_ref(
       source_project.repository.path_to_repo,
       "refs/heads/#{source_branch}",
-      "refs/merge-requests/#{iid}/head"
+      ref_path
     )
   end
 
+  def ref_path
+    "refs/merge-requests/#{iid}/head"
+  end
+
+  def ref_is_fetched?
+    File.exists?(File.join(project.repository.path_to_repo, ref_path))
+  end
+
+  def ensure_ref_fetched
+    fetch_ref unless ref_is_fetched?
+  end
+
   def in_locked_state
     begin
       lock_mr