First run polipo with parent proxy set to Shadowsocks:
apt-get install polipo
service polipo stop
polipo socksParentProxy=localhost:1080
Then you can play with the HTTP proxy:
http_proxy=http://localhost:8123 apt-get update
http_proxy=http://localhost:8123 curl www.google.com
http_proxy=http://localhost:8123 wget www.google.com
git config --global http.proxy 127.0.0.1:8123
git clone https://github.com/xxx/xxx.git
git xxx
git xxx
git config --global --unset-all http.proxy