Stop generating superfluous clock entries

Avoid closing the currently clocking entry when clocking in the same task
again.  Leave the clock entry open until some other task is clocked in.

This allows us to clock in tasks with hooks that are called frequently without
generating lots of short sequential clock entries for the same task.

lisp/org-clock.el

@@ -854,15 +854,29 @@ the clocking selection, associated with the letter `d'."
       (when interrupting
 	;; We are interrupting the clocking of a different task.
 	;; Save a marker to this task, so that we can go back.
+	;; First check if we are trying to clock into the same task!
+	(when (save-excursion
+		(unless selected-task
+		  (org-back-to-heading t))
+		(and (equal (marker-buffer org-clock-hd-marker)
+			    (if selected-task
+				(marker-buffer selected-task)
+			      (current-buffer)))
+		     (= (marker-position org-clock-hd-marker)
+			(if selected-task
+			    (marker-position selected-task)
+			  (point)))))
+	  (message "Clock continues in \"%s\"" org-clock-heading)
+	  (throw 'abort nil))
 	(move-marker org-clock-interrupted-task
 		     (marker-position org-clock-marker)
 		     (marker-buffer org-clock-marker))
 	(org-clock-out t))
-      
+
       (when (equal select '(16))
 	;; Mark as default clocking task
 	(org-clock-mark-default-task))
-      
+
       ;; Clock in at which position?
       (setq target-pos
 	    (if (and (eobp) (not (org-on-heading-p)))