git-proxy: helper script to connect to the Git repository through a SOCKS proxy

Connect to a Git repository at host HOST and port PORT through a SOCKS proxy.

The address of the proxy is read from (in order of priority):
  - the `GIT_SOCKS_PROXY` environment variable
  - the `SOCKS_PROXY` or `SOCKS5_PROXY` environment variable
  - the `ALL_PROXY` environment variable (see [curl(1)](http://curl.haxx.se/docs/manpage.html#ENVIRONMENT))
  - the `socks.proxy` git option
  - the default value: `localhost:1080`