Provide a "differential.changeset.search" Conduit API method
Summary: Ref T13605. Support selecting a diff's changesets (to get a list of affected file paths) via the API.
Test Plan: Called API with no arguments, diffPHIDs, PHIDs, IDs. Got sensible output.
Maniphest Tasks: T13605
Differential Revision: https://secure.phabricator.com/D21558