bug#47673: Absurdly long input field in Customize

[scame]

>
> > Does this edit buffer need to appear so wide? Can't it be displayed shorter?
>
> It deletes everything up to the end of the line.
>

I checked the code, and from a cursory look the problem seems to be that the 
:extend property was added to the widget-field face, that's why every field 
became as wide as the screen, because the newline after the field has this face:

  
https://github.com/emacs-mirror/emacs/commit/b5781e63cbe7e4c24e4eea3a400880857348ad19

this was done to fix this bug:

  https://debbugs.gnu.org/cgi/bugreport.cgi?bug=37774


and apparently no one noticed this change made all customize fields very long.