R1:f38730aab88e

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.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D16158