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.

spec/models/merge_request_spec.rb

@@ -810,30 +810,30 @@ describe MergeRequest do
   end
 
   describe '#can_remove_source_branch?' do
-    let(:user) { create(:user) }
-    let(:user2) { create(:user) }
+    set(:user) { create(:user) }
+    set(:merge_request) { create(:merge_request, :simple) }
 
-    before do
-      subject.source_project.team << [user, :master]
+    subject { merge_request }
 
-      subject.source_branch = "feature"
-      subject.target_branch = "master"
-      subject.save!
+    before do
+      subject.source_project.add_master(user)
     end
 
     it "can't be removed when its a protected branch" do
       allow(ProtectedBranch).to receive(:protected?).and_return(true)
+
       expect(subject.can_remove_source_branch?(user)).to be_falsey
     end
 
     it "can't remove a root ref" do
-      subject.source_branch = "master"
-      subject.target_branch = "feature"
+      subject.update(source_branch: 'master', target_branch: 'feature')
 
       expect(subject.can_remove_source_branch?(user)).to be_falsey
     end
 
     it "is unable to remove the source branch for a project the user cannot push to" do
+      user2 = create(:user)
+
       expect(subject.can_remove_source_branch?(user2)).to be_falsey
     end
 
@@ -844,6 +844,7 @@ describe MergeRequest do
     end
 
     it "cannot be removed if the last commit is not also the head of the source branch" do
+      subject.clear_memoized_shas
       subject.source_branch = "lfs"
 
       expect(subject.can_remove_source_branch?(user)).to be_falsey
@@ -1635,16 +1636,12 @@ 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
+    context 'when using the after_update hook to update' do
+      context 'when the branches are 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)
+          expect { subject.update!(source_branch: subject.target_branch, target_branch: subject.source_branch) }
+            .to change { subject.merge_request_diff.start_commit_sha }
+            .and change { subject.merge_request_diff.head_commit_sha }
         end
       end
     end