sorin-ionescu/prezto — pull request #662 by kylemanna (2014-08-28 04:12:17+00:00)
Title: ssh: Set default SSH_AGENT_PID for grep

* If `SSH_AGENT_PID` is unset, grep will succeed if another ssh-agent is
running as it will match the string ` ssh-agent` where the PID was an
empty string.
* Set a default value for the grep to a value that will never match if
unset, i.e. -1.