Fix %start-line in `org-babel-tangle-comment-links'

* lisp/ob-tangle.el (org-babel-tangle-comment-links): %start-line
reports the line number, not the buffer position at the beginning of
line.

Reported-by: immanuel <immanuel.litzroth@gmail.com>
<http://lists.gnu.org/r/emacs-orgmode/2019-08/msg00267.html>

lisp/ob-tangle.el

@@ -494,14 +494,16 @@ non-nil, return the full association list to be used by
       result)))
 
 (defun org-babel-tangle-comment-links (&optional info)
-  "Return a list of begin and end link comments for the code block at point."
-  (let ((link-data
-	 `(("start-line" . ,(number-to-string
-			     (org-babel-where-is-src-block-head)))
-	   ("file" . ,(buffer-file-name))
-	   ("link" . ,(org-no-properties (org-store-link nil)))
-	   ("source-name" .
-	    ,(nth 4 (or info (org-babel-get-src-block-info 'light)))))))
+  "Return a list of begin and end link comments for the code block at point.
+INFO, when non nil, is the source block information, as returned
+by `org-babel-get-src-block-info'."
+  (let ((link-data (pcase (or info (org-babel-get-src-block-info 'light))
+		     (`(,_ ,_ ,_ ,_ ,name ,start ,_)
+		      `(("start-line" . ,(org-with-point-at start
+					   (line-beginning-position)))
+			("file" . ,(buffer-file-name))
+			("link" . ,(org-no-properties (org-store-link nil)))
+			("source-name" . ,name))))))
     (list (org-fill-template org-babel-tangle-comment-format-beg link-data)
 	  (org-fill-template org-babel-tangle-comment-format-end link-data))))