lib/diffs: mark trailing tabs similar to trailing spaces

So far, spaces have not been marked up, but trailing spaces are followed by
'<i></i>' (newline symbol). Tabs have been marked up as '<u>\t</u>' (underlined
with icon), but trailing tabs didn't get the same "trailing whitespace" markup
as trailing spaces got.

Fix this unfairness by handling trailing tabs explicitly as '<u>\t</u><i></i>'
so they get both kinds of markup.