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

[Juri Linkov]

> some lines are truly added or deleted.  the problem is they look
> exactly like lines that are changed in the other polarity.  so you
> can't tell which they are.
> i don't think it's too heavy to indicate such lines by e.g. inverse
> video on the polarity character -+ or so.  others might.

Maybe adding the refined face only on indicators of the
added/removed hunks could help (if such highlighting won't mislead
into interpreting this as changes only in newlines).