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))
  - the socks.proxy git option
  - the default value: localhost:1080
git-proxy: improve documentation