bug#35695: 27.0.50; Faces diff-added and diff-removed look the same in a 256 color terminal

[Eli Zaretskii]

> From: Anders Lindgren <andlind@gmail.com>
> Date: Sun, 12 May 2019 07:03:41 +0200
> 
> In Emac 26, they were defined using #ddffdd and #ffdddd, respectively, which 
> is rendered fine in the terminal
> (on both Emacs 26 and 27).
> 
> One way to solve this is to add alternative color definitions using the 
> "min-colors" requirement. Another is to go
> back to the old values.

I think we should go to the back values when min-colors is 256 or
less.