Fix auto filling in a paragraph directly following a comment

* lisp/org.el (org-auto-fill-function): Make sure `adaptive-fill-mode'
  mode is nil when pre-computed `fill-prefix' is the empty string.
  Otherwise filling functions from fill.el think it has to be computed
  again and overwrite it.

lisp/org.el

@@ -21065,7 +21065,10 @@ a footnote definition, try to fill the first paragraph within."
   ;; Check if auto-filling is meaningful.
   (let ((fc (current-fill-column)))
     (when (and fc (> (current-column) fc))
-      (let ((fill-prefix (org-adaptive-fill-function)))
+      (let* ((fill-prefix (org-adaptive-fill-function))
+	     ;; Enforce empty fill prefix, if required.  Otherwise, it
+	     ;; will be computed again.
+	     (adaptive-fill-mode (not (equal fill-prefix ""))))
 	(when fill-prefix (do-auto-fill))))))
 
 (defun org-comment-line-break-function (&optional soft)

testing/lisp/test-org.el

@@ -254,6 +254,22 @@ http://article.gmane.org/gmane.emacs.orgmode/21459/"
 	      (end-of-line)
 	      (org-auto-fill-function)
 	      (buffer-string)))))
+  ;; A hash within a line isn't a comment.
+  (should-not
+   (equal "12345 # 7890\n# 1"
+	  (org-test-with-temp-text "12345 # 7890 1"
+	    (let ((fill-column 12))
+	      (end-of-line)
+	      (org-auto-fill-function)
+	      (buffer-string)))))
+  ;; Correctly interpret empty prefix.
+  (should-not
+   (equal "# a\n# b\nRegular\n# paragraph"
+	  (org-test-with-temp-text "# a\n# b\nRegular paragraph"
+	    (let ((fill-column 12))
+	      (end-of-line 3)
+	      (org-auto-fill-function)
+	      (buffer-string)))))
   ;; Comment block: auto fill contents.
   (should
    (equal "#+BEGIN_COMMENT\n12345\n7890\n#+END_COMMENT"