Fix for puma issue running via `rails s`

A recent update seems to have broken running the server locally
via `bundle exec rails s`. Puma no longer seems to respond to
`#cli_config` in that scenario, so this patch fixes that situation.

lib/gitlab/git/rugged_impl/use_rugged.rb

@@ -16,7 +16,9 @@ module Gitlab
         end
 
         def running_puma_with_multiple_threads?
-          Gitlab::Runtime.puma? && ::Puma.cli_config.options[:max_threads] > 1
+          return false unless Gitlab::Runtime.puma?
+
+          ::Puma.respond_to?(:cli_config) && ::Puma.cli_config.options[:max_threads] > 1
         end
 
         def execute_rugged_call(method_name, *args)

spec/lib/gitlab/git/rugged_impl/use_rugged_spec.rb

@@ -122,6 +122,12 @@ describe Gitlab::Git::RuggedImpl::UseRugged, :seed_helper do
         allow(Gitlab::Runtime).to receive(:puma?).and_return(true)
       end
 
+      it "returns false when Puma doesn't support the cli_config method" do
+        allow(::Puma).to receive(:respond_to?).with(:cli_config).and_return(false)
+
+        expect(subject.running_puma_with_multiple_threads?).to be_falsey
+      end
+
       it 'returns false for single thread Puma' do
         allow(::Puma).to receive_message_chain(:cli_config, :options).and_return(max_threads: 1)