Replace legacy Differential queries for "open" revisions with a modern mechanism

Summary: Ref T2543. Several queries want only open revisions. Provide a tailored, non-legacy way to issue that query.

Test Plan: Viewed some of these callsites (e.g., "Similar open revisions affecting these files"), saw only open revisions.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T2543

Differential Revision: https://secure.phabricator.com/D18395