Use "resolve()", not "execute()", for PhutilExecPassthru callsites in Phabricator

Summary: Ref T13660. Clean up callsites to "PhutilExecPassthru->execute()" to prepare to deprecate it.

Test Plan:

• 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.

Reviewers: cspeckmim

Reviewed By: cspeckmim

Maniphest Tasks: T13660

Differential Revision: https://secure.phabricator.com/D21703