Trigger iid logic from GitHub importer for merge requests.

lib/gitlab/github_import/importer/pull_request_importer.rb

@@ -76,7 +76,13 @@ module Gitlab
             merge_request_id = GithubImport
               .insert_and_return_id(attributes, project.merge_requests)
 
-            [project.merge_requests.find(merge_request_id), false]
+            merge_request = project.merge_requests.find(merge_request_id)
+
+            # We use .insert_and_return_id which effectively disables all callbacks.
+            # Trigger iid logic here to make sure we track internal id values consistently.
+            merge_request.ensure_target_project_iid!
+
+            [merge_request, false]
           end
         rescue ActiveRecord::InvalidForeignKey
           # It's possible the project has been deleted since scheduling this

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

@@ -111,6 +111,16 @@ describe Gitlab::GithubImport::Importer::PullRequestImporter, :clean_gitlab_redi
         expect(mr).to be_instance_of(MergeRequest)
         expect(exists).to eq(false)
       end
+
+      it 'triggers internal_id functionality to track greatest iids' do
+        mr = build_stubbed(:merge_request, source_project: project, target_project: project)
+        allow(Gitlab::GithubImport).to receive(:insert_and_return_id).and_return(mr.id)
+        allow(project.merge_requests).to receive(:find).with(mr.id).and_return(mr)
+
+        expect(mr).to receive(:ensure_target_project_iid!)
+
+        importer.create_merge_request
+      end
     end
 
     context 'when the author could not be found' do