org-babel-tangle-single-block: Do not rely on match-data from `org-back-to-heading'

* lisp/ob-tangle.el (org-babel-tangle-single-block): Explicitly move
to the beginning of title instead of assuming that
`org-back-to-heading' sets match data appropriately (it is not
documented to do so).
* testing/lisp/test-ob-tangle.el (ob-tangle/comment-org): Add test.

Reported-by: Nick Dokos <ndokos@redhat.com>
Link: https://orgmode.org/list/87bkh8k3at.fsf@alphaville.usersys.redhat.com

lisp/ob-tangle.el

@@ -571,7 +571,7 @@ non-nil, return the full association list to be used by
 	      (max (condition-case nil
 		       (save-excursion
 			 (org-back-to-heading t) ; Sets match data
-			 (match-end 0))
+			 (re-search-forward org-outline-regexp))
 		     (error (point-min)))
 		   (save-excursion
 		     (if (re-search-backward

testing/lisp/test-ob-tangle.el

@@ -99,6 +99,23 @@ echo 1
                                    (buffer-string)))
         (delete-file "test-ob-tangle.sh"))))))
 
+(ert-deftest ob-tangle/comment-org ()
+  "Test :commends org."
+  (should
+   (string-match
+    (regexp-quote ";; Function heading")
+    (org-test-with-temp-text-in-file
+        "* Function heading
+
+  #+begin_src elisp :tangle \"test-ob-tange.el\" :comments org
+    (message \"FOO\")
+  #+end_src"
+      (unwind-protect
+          (progn (org-babel-tangle)
+                 (with-temp-buffer (insert-file-contents "test-ob-tange.el")
+                                   (buffer-string)))
+        (delete-file "test-ob-tange.el"))))))
+
 (ert-deftest ob-tangle/comment-links-numbering ()
   "Test numbering of source blocks when commenting with links."
   (should