Clear caches before updating MR diffs

The hook ordering influenced the diffs being generated as these used
values from before the update due to the memoization still being in
place. This commit reorders them and tests against this behaviour.

app/models/merge_request.rb

@@ -56,8 +56,8 @@ class MergeRequest < ActiveRecord::Base
   serialize :merge_params, Hash # rubocop:disable Cop/ActiveRecordSerialize
 
   after_create :ensure_merge_request_diff, unless: :importing?
-  after_update :reload_diff_if_branch_changed
   after_update :clear_memoized_shas
+  after_update :reload_diff_if_branch_changed
 
   # When this attribute is true some MR validation is ignored
   # It allows us to close or modify broken merge requests

spec/models/merge_request_spec.rb

@@ -1634,6 +1634,20 @@ describe MergeRequest do
 
       subject.reload_diff
     end
+
+    context 'wehn using the after_update hook to update' do
+      context 'when the merge request is updated' do
+        it 'uses the new heads to generate the diff' do
+          target_branch_sha = subject.target_branch_sha
+          source_branch_sha = subject.source_branch_sha
+
+          subject.update!(source_branch: subject.target_branch, target_branch: subject.source_branch)
+
+          expect(subject.merge_request_diff.start_commit_sha).not_to eq(target_branch_sha)
+          expect(subject.merge_request_diff.head_commit_sha).not_to eq(source_branch_sha)
+        end
+      end
+    end
   end
 
   describe '#update_diff_discussion_positions' do

spec/services/merge_requests/merge_service_spec.rb

 require 'spec_helper'
 
 describe MergeRequests::MergeService do
-  let(:user) { create(:user) }
-  let(:user2) { create(:user) }
+  set(:user) { create(:user) }
+  set(:user2) { create(:user) }
   let(:merge_request) { create(:merge_request, :simple, author: user2, assignee: user2) }
   let(:project) { merge_request.project }
 
   before do
-    project.team << [user, :master]
-    project.team << [user2, :developer]
+    project.add_master(user)
+    project.add_developer(user2)
   end
 
   describe '#execute' do