bug#33567: Syntactic fontification of diff hunks

[Juri Linkov]

> Have you considered bringing over the "default" face foregrounds as well,
> though, not just the special highlighting?
>
> My brain sees syntax highlighting in some places and keeps wanting to
> interpret the green and red text and something syntactically meaningful
> as well.
>
> Maybe we could add extra ancestors for diff-added and diff-removed faces,
> with default foregrounds, and use them for the base highlighting of diff
> chunks, aside from +'s and -'s.
>
> Another thing that messes up the display a bit, is that comments and
> docstrings also have customized backgrounds in my theme, but that's less
> essential and seems harder to fix.

I tried to replicate your settings: in emacs -Q I change the default
foreground with e.g. M-x set-foreground-color RET

A side note: M-x set-foreground-color RET TAB shows a list
of colors with different backgrounds, not foregrounds
as it would be natural to expect, so it is difficult to decide
what foreground color to select by looking at background colors.
This needs a separate bug#.

Ok, let's say I chose M-x set-foreground-color RET LightSeaGreen

Now in diff-mode I see exactly the same fontification with the same
foreground colors and as seen in the original buffer visited from
diff-mode by `C-c C-c', plus red/green background added by diff-mode.