R1:af6cc0c934c4

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…