ob-tangle.el: Fix single block tangle

lisp/ob-tangle.el (org-babel-tangle-single-block): Fix the result when
`only-this-block' is `t' to match what is expected by
`org-babel-tangle'.
(org-babel-effective-tangled-filename): Extract the
computation of filename of tangled src block.
(org-babel-tangle-collect-blocks): Use `org-babel-effective-tangled-filename'.

lisp/ob-tangle.el

@@ -350,6 +350,22 @@ that the appropriate major-mode is set.  SPEC has the form:
 	       (org-fill-template
 		org-babel-tangle-comment-format-end link-data)))))
 
+(defun org-babel-effective-tangled-filename (buffer-fn src-lang src-tfile)
+  "Return effective tangled filename of a source-code block.
+BUFFER-FN is the name of the buffer, SRC-LANG the language of the
+block and SRC-TFILE is the value of the :tangle header argument,
+as computed by `org-babel-tangle-single-block'."
+  (let ((base-name (cond
+                    ((string= "yes" src-tfile)
+                     ;; Use the buffer name
+                     (file-name-sans-extension buffer-fn))
+                    ((> (length src-tfile) 0) src-tfile)))
+        (ext (or (cdr (assoc src-lang org-babel-tangle-lang-exts)) src-lang)))
+    (when base-name
+      ;; decide if we want to add ext to base-name
+      (if (and ext (string= "yes" src-tfile))
+          (concat base-name "." ext) base-name))))
+
 (defun org-babel-tangle-collect-blocks (&optional lang-re tangle-file)
   "Collect source blocks in the current Org file.
 Return an association list of language and source-code block
@@ -378,17 +394,8 @@ be used to limit the collected code blocks by target file."
 	    ;; file name.
 	    (let* ((block (org-babel-tangle-single-block counter))
                    (src-tfile (cdr (assq :tangle (nth 4 block))))
-		   (base-name (cond
-			       ((string= "yes" src-tfile)
-                                ;; buffer name
-				(file-name-sans-extension
-				 (nth 1 block)))
-			       ((> (length src-tfile) 0) src-tfile)))
-		   (ext (or (cdr (assoc src-lang org-babel-tangle-lang-exts)) src-lang))
-		   (file-name (when base-name
-				;; decide if we want to add ext to base-name
-				(if (and ext (string= "yes" src-tfile))
-				    (concat base-name "." ext) base-name)))
+		   (file-name (org-babel-effective-tangled-filename
+                               (nth 1 block) src-lang src-tfile))
 		   (by-fn (assoc file-name blocks)))
 	      (if by-fn (setcdr by-fn (cons (cons src-lang block) (cdr by-fn)))
 		(push (cons file-name (list (cons src-lang block))) blocks)))))))
@@ -482,7 +489,10 @@ non-nil, return the full association list to be used by
 		  (org-trim (org-remove-indentation body)))
 		comment)))
     (if only-this-block
-	(list (cons src-lang (list result)))
+        (let* ((src-tfile (cdr (assq :tangle (nth 4 result))))
+               (file-name (org-babel-effective-tangled-filename
+                           (nth 1 result) src-lang src-tfile)))
+          (list (cons file-name (list (cons src-lang result)))))
       result)))
 
 (defun org-babel-tangle-comment-links (&optional info)