Set new MR diff as latest even when importing

These are already set when `set_latest_merge_request_diff_ids!`
is called in `ProjectTreeRestorer` but we do not use this when doing
GitHub imports. So we need to do this for imports too.

app/models/merge_request_diff.rb

@@ -136,7 +136,7 @@ class MergeRequestDiff < ApplicationRecord
   # All diff information is collected from repository after object is created.
   # It allows you to override variables like head_commit_sha before getting diff.
   after_create :save_git_content, unless: :importing?
-  after_create_commit :set_as_latest_diff, unless: :importing?
+  after_create_commit :set_as_latest_diff
 
   after_save :update_external_diff_store, if: -> { !importing? && saved_change_to_external_diff? }
 

spec/lib/gitlab/github_import/importer/pull_request_importer_spec.rb

@@ -311,10 +311,11 @@ describe Gitlab::GithubImport::Importer::PullRequestImporter, :clean_gitlab_redi
       end
     end
 
-    it 'creates the merge request diffs' do
+    it 'creates a merge request diff and sets it as the latest' do
       mr = insert_git_data
 
       expect(mr.merge_request_diffs.exists?).to eq(true)
+      expect(mr.reload.latest_merge_request_diff_id).to eq(mr.merge_request_diffs.first.id)
     end
 
     it 'creates the merge request diff commits' do

spec/lib/gitlab/import/merge_request_creator_spec.rb

@@ -21,8 +21,11 @@ describe Gitlab::Import::MergeRequestCreator do
 
         subject.execute(attributes)
 
-        expect(merge_request.reload.merge_request_diffs.count).to eq(1)
-        expect(merge_request.reload.merge_request_diffs.first.commits.count).to eq(commits_count)
+        merge_request.reload
+
+        expect(merge_request.merge_request_diffs.count).to eq(1)
+        expect(merge_request.merge_request_diffs.first.commits.count).to eq(commits_count)
+        expect(merge_request.latest_merge_request_diff_id).to eq(merge_request.merge_request_diffs.first.id)
       end
     end