Use default_branch for code coverage

This commits allow to fetch our code coverage data for the
default_branch with our graphql endpoint.

ee/app/graphql/ee/types/project_type.rb

@@ -153,6 +153,7 @@ module EE
             results = ::Ci::DailyBuildGroupReportResult
               .by_projects(project_ids)
               .with_coverage
+              .with_default_branch
               .latest
               .summaries_per_project
 

ee/spec/requests/api/graphql/project/code_coverage_summary_spec.rb

@@ -37,6 +37,7 @@ RSpec.describe 'Getting code coverage summary in a project' do
   end
 
   context 'when project has coverage' do
+    context 'for the default branch' do
       let!(:daily_build_group_report_result) { create(:ci_daily_build_group_report_result, project: project) }
 
       it 'contains code coverage summary data', :aggregates_failures do
@@ -48,6 +49,17 @@ RSpec.describe 'Getting code coverage summary in a project' do
       end
     end
 
+    context 'not for the default branch' do
+      let!(:daily_build_group_report_result) { create(:ci_daily_build_group_report_result, :on_feature_branch, project: project) }
+
+      it 'returns nil' do
+        post_graphql(query, current_user: current_user)
+
+        expect(code_coverage_summary_graphql_data).to be_nil
+      end
+    end
+  end
+
   context 'when project does not have coverage' do
     it 'returns nil' do
       post_graphql(query, current_user: current_user)