Convert a low-level VCS query in Diff extraction to a Conduit call

Summary: Ref T9319. Ref T2783. This won't currently work in a future environment where daemons and repositories are not on the same host. Send it over Conduit instead.

Test Plan: Used bin/differential attach-commit rX Dy to force attachment, saw valid content pull over Conduit.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T2783, T9319

Differential Revision: https://secure.phabricator.com/D14969