Provide "differential.diff.search"

Summary: See PHI90. For now, this only provides a limited amount of information, but should satisfy the use case in PHI90 and build toward a more complete version in the future.

Test Plan: Used new Conduit method to retrieve information about diffs.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18744