git fetch upstream
异常信息：
- error: RPC failed; curl 18 Recv failure: Connection was reset
fatal: expected flush after ref listing - fatal: unable to access 'https://github.com/apache/superset.git/': Failed to connect to github.com port 443 after 21099 ms: Couldn't connect to server
解决方案：
- git config --global --unset http.proxy
- git config --global --unset https.proxy
候选方案：
- 增加缓冲区大小：
git config --global http.postBuffer 524288000
- 尝试使用HTTPS：
git remote set-url upstream https://github.com/apache/superset.git
git fetch upstream