Backport #39887 by @silverwind
Code comments next to a diff hunk could show one line too high in the
conversation view vs. in the code view that showed them on the correct
line.
- Treat the hunk end in `CutDiffAroundLine` as exclusive, so the line
after a hunk falls back to the unchanged-line context
- Don't count `\ No newline at end of file` markers as lines when
cutting the hunk
- Share the unchanged-line fallback with migrated review comments
Co-authored-by: silverwind <me@silverwind.io>