org-agenda: Keep local-variables defining text-scale

* lisp/org-agenda.el (org-agenda-mode): Save and restore local
	   variables text-scale-mode-amount, text-scale-mode,
	   text-scale-mode-lighter, face-remapping-alist.  In effect
	   this keeps the text-scale at agenda rebuild and redo.

This commit is predicated on the newsgroup post of Samuel Wales "point
moves and zoom level reverts when refreshing agenda" at Sun, 27 May
2018.

lisp/org-agenda.el

@@ -2184,29 +2184,37 @@ The following commands are available:
 
 \\{org-agenda-mode-map}"
   (interactive)
-  (cond (org-agenda-doing-sticky-redo
-	 ;; Refreshing sticky agenda-buffer
-	 ;;
-	 ;; Preserve the value of `org-agenda-local-vars' variables,
-	 ;; while letting `kill-all-local-variables' kill the rest
-	 (let ((save (buffer-local-variables)))
-	   (kill-all-local-variables)
+  (let ((agenda-local-vars-to-keep
+	 '(text-scale-mode-amount
+	   text-scale-mode
+	   text-scale-mode-lighter
+	   face-remapping-alist))
+	(save (buffer-local-variables)))
+    (kill-all-local-variables)
+    (cond (org-agenda-doing-sticky-redo
+	   ;; Refreshing sticky agenda-buffer
+	   ;;
+	   ;; Preserve the value of `org-agenda-local-vars' variables.
 	   (mapc #'make-local-variable org-agenda-local-vars)
 	   (dolist (elem save)
 	     (pcase elem
 	       (`(,var . ,val)		;ignore unbound variables
 		(when (and val (memq var org-agenda-local-vars))
-		  (set var val))))))
-	 (setq-local org-agenda-this-buffer-is-sticky t))
-	(org-agenda-sticky
-	 ;; Creating a sticky Agenda buffer for the first time
-	 (kill-all-local-variables)
-	 (mapc 'make-local-variable org-agenda-local-vars)
-	 (setq-local org-agenda-this-buffer-is-sticky t))
-	(t
-	 ;; Creating a non-sticky agenda buffer
-	 (kill-all-local-variables)
-	 (setq-local org-agenda-this-buffer-is-sticky nil)))
+		  (set var val)))))
+	   (setq-local org-agenda-this-buffer-is-sticky t))
+	  (org-agenda-sticky
+	   ;; Creating a sticky Agenda buffer for the first time
+	   (mapc 'make-local-variable org-agenda-local-vars)
+	   (setq-local org-agenda-this-buffer-is-sticky t))
+	  (t
+	   ;; Creating a non-sticky agenda buffer
+	   (setq-local org-agenda-this-buffer-is-sticky nil)))
+    (mapc #'make-local-variable agenda-local-vars-to-keep)
+    (dolist (elem save)
+      (pcase elem
+	(`(,var . ,val)		;ignore unbound variables
+	 (when (and val (memq var agenda-local-vars-to-keep))
+	   (set var val))))))
   (setq org-agenda-undo-list nil
 	org-agenda-pending-undo-list nil
 	org-agenda-bulk-marked-entries nil)