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.