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