Only skip directories in lgrep, not the other grep commands

* lisp/progmodes/grep.el (lgrep): Do the directory ignores here
(bug#23590).
(grep-compute-defaults): ... instead of here, because this would
affect all grep commands, not just lgrep.

etc/NEWS

@@ -545,6 +545,11 @@ grep emits SGR ANSI escape sequences to color its output.  The new variable
 markers in order to provide highlighting in the source buffer.  The variable
 can be customized to accommodate other grep-like tools.
 
+---
+*** The 'lgrep' command now ignores directories.
+On systems where the grep command supports it, directories will be
+skipped.
+
 ** Help
 
 +++

lisp/progmodes/grep.el

@@ -703,11 +703,6 @@ The value depends on `grep-command', `grep-template',
       (let ((grep-options
 	     (concat (if grep-use-null-device "-n" "-nH")
                      (if grep-use-null-filename-separator " --null")
-                     (when (grep-probe grep-program
-                                       `(nil nil nil "--directories=skip" "foo"
-                                             ,null-device)
-                                       nil 1)
-                       " --directories=skip")
                      (when (grep-probe grep-program
                                        `(nil nil nil "-e" "foo" ,null-device)
                                        nil 1)
@@ -1126,6 +1121,11 @@ command before it's run."
 				     grep-find-ignored-files
 				     " --exclude=")))))
 	(when command
+          (when (grep-probe grep-program
+                            `(nil nil nil "--directories=skip" "foo"
+                                  ,null-device)
+                            nil 1)
+            (setq command (concat command " --directories=skip")))
 	  (if confirm
 	      (setq command
 		    (read-from-minibuffer "Confirm: "