Memoize Git::Repository#has_visible_content?

This is called repeatedly when viewing a merge request, and this should
improve performance significantly by avoiding shelling out to git every time.

This should help https://gitlab.com/gitlab-com/infrastructure/issues/4027.

changelogs/unreleased/sh-memoize-repository-empty.yml

+---
+title: Memoize Git::Repository#has_visible_content?
+merge_request:
+author:
+type: performance

lib/gitlab/git/repository.rb

@@ -9,6 +9,7 @@ module Gitlab
       include Gitlab::Git::RepositoryMirroring
       include Gitlab::Git::Popen
       include Gitlab::EncodingHelper
+      include Gitlab::Utils::StrongMemoize
       ALLOWED_OBJECT_DIRECTORIES_VARIABLES = %w[
         GIT_OBJECT_DIRECTORY
@@ -232,6 +233,12 @@ module Gitlab
       end
       def has_local_branches?
+        strong_memoize(:has_local_branches) do
+          uncached_has_local_branches?
+        end
+      end
+
+      def uncached_has_local_branches?
         gitaly_migrate(:has_local_branches, status: Gitlab::GitalyClient::MigrationStatus::OPT_OUT) do |is_enabled|
           if is_enabled
             gitaly_repository_client.has_local_branches?

spec/lib/gitlab/git/repository_spec.rb

@@ -463,7 +463,7 @@ describe Gitlab::Git::Repository, seed_helper: true do
         it 'returns false when there are no branches' do
           # Sanity check
-          expect(repository.has_local_branches?).to eq(true)
+          expect(repository.uncached_has_local_branches?).to eq(true)
           FileUtils.rm_rf(File.join(repository.path, 'packed-refs'))
           heads_dir = File.join(repository.path, 'refs/heads')
@@ -473,6 +473,16 @@ describe Gitlab::Git::Repository, seed_helper: true do
           expect(repository.has_local_branches?).to eq(false)
         end
       end
+
+      context 'memoizes the value' do
+        it 'returns true' do
+          expect(repository).to receive(:uncached_has_local_branches?).once.and_call_original
+
+          2.times do
+            expect(repository.has_local_branches?).to eq(true)
+          end
+        end
+      end
     end
     context 'with gitaly' do