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