Ref T9846. This rewrites the rendering algorithm in a mostly-compatible way and fixes the major issue.
- Includes test coverage for removing a newline, from T12765.
- Includes test coverage for mangling an XML tag, from T9846.
This omits two features, which I'll port forward separately:
- For one-line patches, highlighting the patched section.
- For zero-line patches, putting a little caret ("^") under the character where the warning occurred.
I'll restore these features in a followup change.