bug#30559: 26.0.91; grep-use-null-filename-separator customization

[Juri Linkov <juri@linkov.net>]

This is a request to push the patch to the emacs-26 release branch.

Recipe:
1. ‘M-x customize-variable RET grep-use-null-filename-separator RET’
2. Select a non-default value
3. ‘C-x C-s’

Customizing and saving the new defcustom ‘grep-use-null-filename-separator’
fails because ‘grep-host-defaults-alist’ is missing from the second
‘add-to-list’:

diff --git a/lisp/progmodes/grep.el b/lisp/progmodes/grep.el
index 14e251e..755c9c7 100644
--- a/lisp/progmodes/grep.el
+++ b/lisp/progmodes/grep.el
@@ -734,6 +734,8 @@ grep-compute-defaults
 	     (grep-use-null-device ,grep-use-null-device)
 	     (grep-find-command ,grep-find-command)
 	     (grep-find-template ,grep-find-template)
+	     (grep-use-null-filename-separator
+	      ,grep-use-null-filename-separator)
 	     (grep-find-use-xargs ,grep-find-use-xargs)
 	     (grep-highlight-matches ,grep-highlight-matches))))))