Make gnus-newsgroup-name local var available to gnus-summary-mode

* lisp/gnus/gnus-sum.el (gnus-summary-mode-group): New defvar for
  passing gnus-newsgroup-name (a local variable) into the body of
  gnus-summary-mode, so that it's available to gnus-summary-mode-hook.

lisp/gnus/gnus-sum.el

@@ -3064,6 +3064,7 @@ The following commands are available:
   (let ((gnus-summary-local-variables gnus-newsgroup-variables))
     (gnus-summary-make-local-variables))
   (gnus-summary-make-local-variables)
+  (setq gnus-newsgroup-name gnus-summary-mode-group)
   (when (gnus-visual-p 'summary-menu 'menu)
     (gnus-summary-make-menu-bar)
     (gnus-summary-make-tool-bar))
@@ -3457,6 +3458,11 @@ display only a single character."
 					       (point)
 					       (current-buffer))))))
 
+(defvar gnus-summary-mode-group nil
+  "Variable for communication with `gnus-summary-mode'.
+Allows the `gnus-newsgroup-name' local variable to be set before
+the summary mode hooks are run.")
+
 (defun gnus-summary-setup-buffer (group)
   "Initialize summary buffer for GROUP.
 This function does all setup work that relies on the specific
@@ -3476,10 +3482,10 @@ Returns non-nil if the setup was successful."
 	  (not gnus-newsgroup-prepared))
       (set-buffer (gnus-get-buffer-create buffer))
       (setq gnus-summary-buffer (current-buffer))
-      (gnus-summary-mode)
+      (let ((gnus-summary-mode-group group))
+       (gnus-summary-mode))
       (when (gnus-group-quit-config group)
 	(set (make-local-variable 'gnus-single-article-buffer) nil))
-      (setq gnus-newsgroup-name group)
       (turn-on-gnus-mailing-list-mode)
       ;; These functions don't currently depend on GROUP, but might in
       ;; the future.