Skip remote mirror fetch when using ls-remote

When the `gitaly_ruby_remote_branches_ls_remote` feature flag is
enabled, Gitaly will gather remote branches from the mirror via
`ls-remote`, so a fetch is unnecessary.

See https://gitlab.com/gitlab-org/gitaly/-/issues/2670.

app/services/projects/update_remote_mirror_service.rb

@@ -27,7 +27,11 @@ module Projects
       remote_mirror.update_start!
 
       remote_mirror.ensure_remote!
-      repository.fetch_remote(remote_mirror.remote_name, ssh_auth: remote_mirror, no_tags: true)
+
+      # https://gitlab.com/gitlab-org/gitaly/-/issues/2670
+      if Feature.disabled?(:gitaly_ruby_remote_branches_ls_remote)
+        repository.fetch_remote(remote_mirror.remote_name, ssh_auth: remote_mirror, no_tags: true)
+      end
 
       response = remote_mirror.update_repository
 

spec/services/projects/update_remote_mirror_service_spec.rb

@@ -10,6 +10,10 @@ describe Projects::UpdateRemoteMirrorService do
 
   subject(:service) { described_class.new(project, project.creator) }
 
+  before do
+    stub_feature_flags(gitaly_ruby_remote_branches_ls_remote: false)
+  end
+
   describe '#execute' do
     subject(:execute!) { service.execute(remote_mirror, 0) }
 
@@ -102,6 +106,19 @@ describe Projects::UpdateRemoteMirrorService do
         expect(remote_mirror.last_error).to include("refs/heads/develop")
       end
     end
+
+    # https://gitlab.com/gitlab-org/gitaly/-/issues/2670
+    context 'when `gitaly_ruby_remote_branches_ls_remote` is enabled' do
+      before do
+        stub_feature_flags(gitaly_ruby_remote_branches_ls_remote: true)
+      end
+
+      it 'does not perform a fetch' do
+        expect(project.repository).not_to receive(:fetch_remote)
+
+        execute!
+      end
+    end
   end
 
   def stub_fetch_remote(project, remote_name:, ssh_auth:)