flutter upgrade遇到LibreSSL SSL_re

在执行flutter upgrade命令时经常会遇到以下两个错误：

ProcessException: Process exited abnormally:
fatal: unable to access 'https://github.com/flutter/flutter.git/': LibreSSL SSL_read: SSL_ERROR_SYSCALL, errno 54
  Command: git fetch --tags

或者是

ProcessException: Process exited abnormally:
fatal: unable to access 'https://github.com/flutter/flutter.git/': LibreSSL SSL_connect: SSL_ERROR_SYSCALL in connection to github.com:443
  Command: git fetch --tags

错误的原因：

• 是我们在~/.gitconfig文件中配置的remote.origin.proxy参数导致的

解决方法：

git config --global --add remote.origin.proxy ""