Fix bug when computing clock tables.

* lisp/org-clock.el (org-dblock-write:clocktable): Fix bug
when computing clock tables.

Ouch, bad bug.  Resetting the clock file time was always
happening in the same file, not in the different files
visited by the summer.

lisp/org-clock.el

@@ -1839,8 +1839,8 @@ the currently selected interval size."
 	    (setq p1 (plist-put p1 :scope 'file))
 	    (org-prepare-agenda-buffers files)
 	    (while (setq file (pop files))
-	      (setq org-clock-file-total-minutes 0)
 	      (with-current-buffer (find-buffer-visiting file)
+		(setq org-clock-file-total-minutes 0)
 		(setq tbl1 (org-dblock-write:clocktable p1))
 		(when tbl1
 		  (push (org-clocktable-add-file