Fix filling with `adaptive-fill-regexp'

* lisp/org.el (org-adaptive-fill-function): Look for a fill prefix at
  the beginning of the paragraph and subsquently on its second line
  instead of the current line.
* testing/lisp/test-org.el: Add test

lisp/org.el

@@ -22163,7 +22163,15 @@ matches in paragraphs or comments, use it."
 			 (make-string (org-list-item-body-column
 				       (org-element-property :begin parent))
 				      ? ))
-			((looking-at adaptive-fill-regexp) (match-string 0))
+			((and adaptive-fill-regexp
+			      ;; Locally disable
+			      ;; `adaptive-fill-function' to let
+			      ;; `fill-context-prefix' handle
+			      ;; `adaptive-fill-regexp' variable.
+			      (let (adaptive-fill-function)
+				(fill-context-prefix
+				 post-affiliated
+				 (org-element-property :end element)))))
 			((looking-at "[ \t]+") (match-string 0))
 			(t  "")))))
 	     (comment-block

testing/lisp/test-org.el

@@ -273,13 +273,30 @@
 	      (buffer-string)))))
   ;; Auto fill paragraph when `adaptive-fill-regexp' matches.
   (should
-   (equal "> 12345\n> 7890"
+   (equal "> 12345\n  7890"
 	  (org-test-with-temp-text "> 12345 7890"
-	    (let ((fill-column 5)
-		  (adaptive-fill-regexp "[ \t]*>+[ \t]*"))
+	    (let ((fill-column 10)
+		  (adaptive-fill-regexp "[ \t]*>+[ \t]*")
+		  (adaptive-fill-first-line-regexp "\\`[ 	]*\\'"))
 	      (end-of-line)
 	      (org-auto-fill-function)
 	      (buffer-string)))))
+  (should
+   (equal "> 12345\n> 12345\n> 7890"
+	  (org-test-with-temp-text "> 12345\n> 12345 7890"
+	    (let ((fill-column 10)
+		  (adaptive-fill-regexp "[ \t]*>+[ \t]*"))
+	      (goto-char (point-max))
+	      (org-auto-fill-function)
+	      (buffer-string)))))
+  (should-not
+   (equal " 12345\n *12345\n *12345"
+	  (org-test-with-temp-text " 12345\n *12345 12345"
+	    (let ((fill-column 10)
+		  (adaptive-fill-regexp "[ \t]*>+[ \t]*"))
+	      (goto-char (point-max))
+	      (org-auto-fill-function)
+	      (buffer-string)))))
   ;; Auto fill comments.
   (should
    (equal "  # 12345\n  # 7890"