bug#61396: diff mode could distinguish changed from deleted lines

[Juri Linkov]

> a fix is to have a different face for changed lines.  i
> suggest a muted bg face.  another fix is to stick a colored
> marker INDICATOR in A where changes in B exist.
>
>   -now is the time^
>   +now is the time FOR ALL GOOD MEN
>
> where ^ is a colored marker -- actually just a changed bg
> for the newline in this case would work, but i don't think
> emacs supports that.

We have a dedicated face to indicate the changed lines.
Its name is 'diff-changed' and it can be used for context diffs
that have a special indicator '!' at the beginning of changed lines.
Is it possible to output the same indicator for unified diffs?