bug#37774: 27.0.50; new :extend attribute broke visuals of all themes and other packages

[Eli Zaretskii]

> From: Andrey Orst <andreyorst@gmail.com>
> Date: Thu, 17 Oct 2019 16:40:49 +0300
> Cc: 37774@debbugs.gnu.org
> 
> Git diff is an inline diff. It doesn't need changes to be aligned side by 
> side,
> and doesn't need changes to be linked as in side by side diff tools like,
> meld, and diff-viewers in Intellij Idea[1], VSCode[2], Sublime Merge[3], 
> Smartgit[4],
> 
> [1] 
> https://www.oreilly.com/library/view/intellij-idea-essentials/9781784396930/graphics/6930OT_03_44.jpg
> [2] https://www.meziantou.net/assets/result1.png?v=2c344912
> [3] https://edge.alluremedia.com.au/m/l/2018/09/smerge.png
> [4] 
> https://www.syntevo.com/assets/images/products/smartgit/opener-481720cb.png
> 
> and even when we looking at inline diff,
> most GUI tools use extended highlighters,
> because it is easy to understand:
> 
> sublime merge: https://i.ytimg.com/vi/ZrdkEBJV660/maxresdefault.jpg
> Magit: https://magit.vc/screenshots/commit-top.png
> 
> Consider looking at how magit supposed to look in official screenshots here,
> and how it looks after the change. 
> https://emacsair.me/2017/09/01/magit-walk-through/

So maybe a few more faces need to be extended, at least optionally.
The issue that was brought up here was, AFAIU, that this feature
breaks many features so badly that it needs to be reverted or at least
turned off by default.  I have yet to see some sound arguments for
that.