Handle "-p port" flag in ssh-connect

Summary:
The documentation is explicit that Git does not pass this flag:

The $GIT_SSH command will be given exactly two arguments: the username@host (or just host) from the URL and the shell command to execute on that remote system.

This isn't true; it does. Accommodate it.

I'll see if I can fix this in the upstream, too.

Test Plan: Ran various ssh-connect commands with -p, etc.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D7680