Ref T13660. Clean up callsites to "PhutilExecPassthru->execute()" to prepare to deprecate it.
- Grepped for "PhutilExecPassthru" and looked for callsites.
- Ran GIT_SSH=.../ssh-connect git ls-remote origin to execute the "ssh-connect" code.
- The two passthru future methods have no callers and could possibly be removed, but I'm just letting sleeping dogs lie for now.