Remove visibility cycling from general clock mapping function

It's only necessary when editing the buffer, not when only traversing it.  Moved
it to the only function where this is actually done, and added a comment to the
docstring of `timeline-tools-map-clocklines' to remind everyone that editing the
buffer through this function may yield funny surprises :)

site-lisp/timeline-tools.el

@@ -166,14 +166,13 @@ ending time as arguments and point on the beginning of the line.
 For each headline, call HEADLINE-FN with no arguments and point
 on the start of the headline.  Traversal will be done from the
 end of the file upwards.  If the buffer is narrowed, only this
-region will be traversed."
+region will be traversed.  If either CLOCKLINE-FN or HEADLINE-FN
+edit the current buffer, make sure to use `org-show-all' to show
+all insivible elements; otherwise editing may result in
+unpredictable behavior."
   (unless (eq major-mode 'org-mode)
     (user-error "Not in Org mode buffer, cannot parse clocklines"))
 
-  ;; Make sure everything is visible, as otherwise editing may produce odd
-  ;; results
-  (org-show-all)
-
   (let* ((re (concat "^\\(\\*+\\)[ \t]\\|^[ \t]*"
                      org-clock-string
                      "[ \t]*\\(\\[.*?\\]\\)-+\\(\\[.*?\\]\\)")))
@@ -642,6 +641,10 @@ clock line."
         (new-end   (float-time end)))
     (dolist (buffer buffers)
       (with-current-buffer buffer
+        ;; Make sure everything is visible, as otherwise editing may produce odd
+        ;; results
+        (org-show-all)
+
         (timeline-tools-map-clocklines
          (lambda (timestamp-1 timestamp-2)
            (let ((current-start (org-time-string-to-seconds timestamp-1))