In "phutil_passthru()", "resolve()" the future rather than calling "execute()" directly

Summary:
See PHI1862. This code calls "execute()" on the future directly, but that skips some steps -- notably, ServiceProfiler hooks.

Call "resolve()", which has the same effect but includes desirable/expected side effects.

Test Plan: Changed a workflow to run "phutil_passthru('ls')", ran it with "--trace". Before: no execution in trace; after: execution in trace.

Differential Revision: https://secure.phabricator.com/D21470