Improve retrieving default branch of empty repos

When retrieving the default branch of an empty repo, we skip calling
root_ref. This saves us 1 Redis and 1 Gitaly call.

Changelog: performance

app/models/concerns/has_repository.rb

@@ -72,12 +72,10 @@ module HasRepository
   end
 
   def default_branch
-    @default_branch ||= repository.root_ref || default_branch_from_preferences
+    @default_branch ||= repository.empty? ? default_branch_from_preferences : repository.root_ref
   end
 
   def default_branch_from_preferences
-    return unless empty_repo?
-
     (default_branch_from_group_preferences || Gitlab::CurrentSettings.default_branch_name).presence
   end
 

ee/spec/graphql/resolvers/scan_execution_policy_resolver_spec.rb

@@ -14,7 +14,7 @@ RSpec.describe Resolvers::ScanExecutionPolicyResolver do
   let(:policy) { build(:scan_execution_policy, name: 'Run DAST in every pipeline') }
   let(:policy_yaml) { build(:scan_execution_policy_yaml, policies: [policy]) }
 
-  let(:repository) { instance_double(Repository, root_ref: 'master') }
+  let(:repository) { instance_double(Repository, root_ref: 'master', empty?: false) }
 
   describe '#resolve' do
     subject(:resolve_scan_policies) { resolve(described_class, obj: project, ctx: { current_user: user }) }

ee/spec/lib/ee/gitlab/checks/push_rule_check_spec.rb

@@ -101,6 +101,7 @@ RSpec.describe EE::Gitlab::Checks::PushRuleCheck do
 
       it "sets the git env correctly for all hooks", :request_store do
         expect(Gitaly::Repository).to receive(:new)
+                                        .at_least(:once)
                                         .with(a_hash_including(git_object_directory: "objects"))
                                         .and_call_original
 

ee/spec/models/security/orchestration_policy_configuration_spec.rb

@@ -10,7 +10,7 @@ RSpec.describe Security::OrchestrationPolicyConfiguration do
   end
 
   let(:default_branch) { security_policy_management_project.default_branch }
-  let(:repository) { instance_double(Repository, root_ref: 'master') }
+  let(:repository) { instance_double(Repository, root_ref: 'master', empty?: false) }
   let(:policy_yaml) { build(:scan_execution_policy_yaml, policies: [build(:scan_execution_policy, name: 'Run DAST in every pipeline')]) }
 
   before do

lib/gitlab/cache/ci/project_pipeline_status.rb

@@ -69,7 +69,7 @@ module Gitlab
 
           self.sha = commit.sha
           self.status = commit.status
-          self.ref = project.default_branch
+          self.ref = project.repository.root_ref
         end
 
         # We only cache the status for the HEAD commit of a project
@@ -79,7 +79,7 @@ module Gitlab
           return unless sha
           return unless ref
 
-          if commit.sha == sha && project.default_branch == ref
+          if commit.sha == sha && project.repository.root_ref == ref
             store_in_cache
           end
         end

spec/lib/gitlab/import_export/snippet_repo_restorer_spec.rb

@@ -29,6 +29,9 @@ RSpec.describe Gitlab::ImportExport::SnippetRepoRestorer do
           expect(restorer.restore).to be_truthy
         end.to change { SnippetRepository.count }.by(1)
 
+        snippet.repository.expire_method_caches(%i(exists?))
+        expect(snippet.repository_exists?).to be_truthy
+
         blob = snippet.repository.blob_at(snippet.default_branch, snippet.file_name)
         expect(blob).not_to be_nil
         expect(blob.data).to eq(snippet.content)

spec/models/snippet_repository_spec.rb

@@ -115,6 +115,7 @@ RSpec.describe SnippetRepository do
         allow(snippet).to receive(:repository).and_return(repo)
         allow(repo).to receive(:ls_files).and_return([])
         allow(repo).to receive(:root_ref).and_return('master')
+        allow(repo).to receive(:empty?).and_return(false)
       end
 
       it 'infers the commit action based on the parameters if not present' do

spec/workers/post_receive_spec.rb

@@ -428,7 +428,7 @@ RSpec.describe PostReceive do
           end
 
           it 'expires the status cache' do
-            expect(snippet.repository).to receive(:empty?).and_return(true)
+            expect(snippet.repository).to receive(:empty?).at_least(:once).and_return(true)
             expect(snippet.repository).to receive(:expire_status_cache)
 
             perform