Summary:
Ref T13161. See D20181. This allows the intraline highlighter to accept new ">" and "<" spans and apply a different style for them.

The input pattern is `list<segment>`. Each segment is `pair<wild kind, int byte_length>`, i.e. wrap the next `byte_length` bytes in a span of kind `kind`.

Before this change, the possible kinds of segements are `0` (no intraline diff, do not highlight) or `1` (intraline diff, highlight in bright color).

D20181 adds `<` (depth decreased) and `>` (depth increased). These are like `1`, but add a different class so the UI can handle them differently.

Test Plan: See D20181.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13161

Differential Revision: https://secure.phabricator.com/D20182