Reload merge request association reload when update code

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/models/merge_request.rb

@@ -246,13 +246,7 @@ class MergeRequest < ActiveRecord::Base
   end
 
   def diff_refs
-    return unless diff_start_commit || diff_base_commit
-
-    Gitlab::Diff::DiffRefs.new(
-      base_sha:  diff_base_sha,
-      start_sha: diff_start_sha,
-      head_sha:  diff_head_sha
-    )
+    merge_request_diff.diff_refs
   end
 
   def validate_branches
@@ -306,6 +300,7 @@ class MergeRequest < ActiveRecord::Base
 
     old_diff_refs = self.diff_refs
     create_merge_request_diff
+    merge_request_diffs.reload
     new_diff_refs = self.diff_refs
 
     update_diff_notes_positions(

spec/models/merge_request_spec.rb

@@ -291,7 +291,7 @@ describe MergeRequest, models: true do
     end
 
     it "can be removed if the last commit is the head of the source branch" do
-      allow(subject.source_project).to receive(:commit).and_return(subject.diff_head_commit)
+      allow(subject).to receive(:source_branch_head).and_return(subject.diff_head_commit)
 
       expect(subject.can_remove_source_branch?(user)).to be_truthy
     end
@@ -655,7 +655,7 @@ describe MergeRequest, models: true do
     let(:commit) { subject.project.commit(sample_commit.id) }
 
     it "does not change existing merge request diff" do
-      expect(subject.merge_request_diff).not_to receive(:reload_content)
+      expect(subject.merge_request_diff).not_to receive(:save_git_content)
       subject.reload_diff
     end
 
@@ -669,7 +669,6 @@ describe MergeRequest, models: true do
       # Update merge_request_diff so that #diff_refs will return commit.diff_refs
       allow(subject).to receive(:create_merge_request_diff) do
         subject.merge_request_diffs.create(
-          importing: true,
           base_commit_sha: commit.parent_id,
           start_commit_sha: commit.parent_id,
           head_commit_sha: commit.sha