Comment 4 for bug 804641

A snippet from 90gpg-agent reads like this.

   # Invoking gpg-agent with no arguments exits successfully if the agent
   # is already running as pointed by $GPG_AGENT_INFO
   if ! $GPGAGENT 2>/dev/null; then
       STARTUP="$GPGAGENT --daemon --sh --write-env-file=$PID_FILE $STARTUP"
   fi

That's why we need to export GPG_AGENT_INFO; otherwise, the code doesn't even do what it says it does.