Provide better parsing primitives for hunks

Summary:
Ref T1266. This prepares to fix case (2) on T1266 by improving the robustness of hunk parsing.

In particular, the copy detection code abuses this API because it isn't currently expressive or flexible enough.

Make it more flexible and cover it exhaustively.

I'll move callsites to the new stuff in upcoming revisions.

Test Plan: Unit tests.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T1266

Differential Revision: https://secure.phabricator.com/D12144