Diffusion - move search (grep) system commands to happen over Conduit

Summary: Ref T2784.

Test Plan: loaded up my git and mercurial copies of Phabricator. Searched for "diff". Observed many results and pagination working correctly.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2784

Differential Revision: https://secure.phabricator.com/D5955