repository: Convert #fetch_upstream to use in-memory remotes

Convert `Repository#fetch_upstream()` to use `fetch_remote()` with an
in-memory remote. This allows us to skip the call to `add_remote()`: its
backing RPC call is deprecated and will eventually be removed by Gitaly.

The change reuses the `fetch_remote_params` feature flag which is also
used by the GitHub importer and by `fetch_as_mirror()`. This is fine
given that the feature flag hasn't yet been rolled out to production and
is thus still disabled.

ee/app/models/ee/repository.rb

@@ -34,6 +34,17 @@ module EE
     end
 
     def fetch_upstream(url, forced: false, check_tags_changed: false)
+      if ::Feature.enabled?(:fetch_remote_params, project, default_enabled: :yaml)
+        return fetch_remote(
+          MIRROR_REMOTE,
+          url: url,
+          refmap: ["+refs/heads/*:refs/remotes/#{MIRROR_REMOTE}/*"],
+          ssh_auth: project&.import_data,
+          forced: forced,
+          check_tags_changed: check_tags_changed
+        )
+      end
+
       add_remote(MIRROR_REMOTE, url)
 
       fetch_remote(MIRROR_REMOTE, ssh_auth: project&.import_data, forced: forced, check_tags_changed: check_tags_changed)

ee/spec/models/repository_spec.rb

@@ -49,6 +49,41 @@ RSpec.describe Repository do
     end
   end
 
+  describe '#fetch_upstream' do
+    let(:url) { "http://example.com" }
+
+    context 'when :fetch_remote_params is enabled' do
+      it 'fetches the URL without creating a remote' do
+        expect(repository).not_to receive(:add_remote)
+        expect(repository)
+          .to receive(:fetch_remote)
+          .with(described_class::MIRROR_REMOTE, url: url, refmap: ['+refs/heads/*:refs/remotes/upstream/*'], ssh_auth: nil, forced: true, check_tags_changed: true)
+          .and_return(nil)
+
+        repository.fetch_upstream(url, forced: true, check_tags_changed: true)
+      end
+    end
+
+    context 'when :fetch_remote_params is disabled' do
+      before do
+        stub_feature_flags(fetch_remote_params: false)
+      end
+
+      it 'creates a remote and fetches it' do
+        expect(repository)
+          .to receive(:add_remote)
+          .with(described_class::MIRROR_REMOTE, url)
+          .and_return(nil)
+        expect(repository)
+          .to receive(:fetch_remote)
+          .with(described_class::MIRROR_REMOTE, ssh_auth: nil, forced: true, check_tags_changed: true)
+          .and_return(nil)
+
+        repository.fetch_upstream(url, forced: true, check_tags_changed: true)
+      end
+    end
+  end
+
   describe '#with_config' do
     let(:rugged) { rugged_repo(repository) }
     let(:entries) do