Allow Herald text list fields to use "Exists" / "Does not exist" conditions
Summary: This makes it more natural to write Herald rules about commits that appear on any or no branches.
Test Plan: Wrote a commit rule for commits on any branch, ran it with bin/repository reparse --herald <commit>, saw expected results in web UI.
Reviewed By: chad
Differential Revision: https://secure.phabricator.com/D16158