Before, diff file `higlighted_diff_lines`/`parallel_diff_lines` and
diff discussion `truncated_diff_lines` were inconsistent: `text` and
`rich_text` on the latter included the leading +/-/<space> character,
like on the backend, while the former had no `text` and its `rich_text`
had dropped this char.

This resulted in a bug when the suggestions feature expected these diff
line objects to be identical in format and thus interchangeable, which
was not the case.