Simplify Herald logic for loading Differential changes

Summary: Ref T4045. These three methods are fairly copy-pastey. Provide a more formal DifferentialHunk API for querying various types of line ranges.

Test Plan: Used test console to verify that "added content", "removed content", and "changed content" rules still produce the same data.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4045

Differential Revision: https://secure.phabricator.com/D8764