Fix SSH agent in reattached tmux session shells

A new remote logon session creates a different SSHAUTHSOCK and friends. This can be a problem if existing shells in a detached tmux session retain the old ssh-agent environment variables; github pushes and pulls won't work, ssh to other machines will have trouble, etc

Add this function to your shell (works for zsh, I think it will work in bash), and run "fixssh" in each shell that needs to communicate with ssh-agent:

fixssh() {
  for key in SSH_AUTH_SOCK SSH_CONNECTION SSH_CLIENT; do
    if (tmux show-environment | grep "^${key}" > /dev/null); then
      value=`tmux show-environment | grep "^${key}" | sed -e "s/^[A-Z_]*=//"`
      export ${key}="${value}"
    fi
  done
}

EDIT, much later: The following gist lays out a more automatic way of solving this problem: https://gist.github.com/martijnvermaat/8070533