Prefer to run find and grep in parallel in rgrep (bug#71094)

* lisp/progmodes/grep.el (grep-compute-defaults): Prefer `gnu' for
grep-find-use-xargs over `exec-plus', but not on Windows.  (bug#71094)

lisp/progmodes/grep.el

@@ -827,15 +827,23 @@ The value depends on `grep-command', `grep-template',
 	(unless grep-find-use-xargs
 	  (setq grep-find-use-xargs
 		(cond
-		 ((grep-probe find-program
+                 ;; For performance, we want:
-			      `(nil nil nil ,(null-device) "-exec" "echo"
+                 ;; A. Run grep on batches of files (instead of one grep per file)
-				    "{}" "+"))
+                 ;; B. If the directory is large and we need multiple batches,
-		  'exec-plus)
+                 ;;    run find in parallel with a running grep.
+                 ;; "find | xargs grep" gives both A and B
 		 ((and
+                   (not (eq system-type 'windows-nt))
 		   (grep-probe
                     find-program `(nil nil nil ,(null-device) "-print0"))
 		   (grep-probe xargs-program '(nil nil nil "-0" "echo")))
 		  'gnu)
+                 ;; "find -exec {} +" gives A but not B
+		 ((grep-probe find-program
+			      `(nil nil nil ,(null-device) "-exec" "echo"
+				    "{}" "+"))
+		  'exec-plus)
+                 ;; "find -exec {} ;" gives neither A nor B.
 		 (t
 		  'exec))))
 	(unless grep-find-command