org-refile: Optionally prefix refile targets with buffer name

* org.el (org-refile-get-targets): Add case to optionally prefix
refile targets with the buffer name.
(org-refile-use-outline-path): Add new option setting and document.

Having an option to use the buffer name as a prefix is convenient,
since this will work hand in hand with uniquify to only show those
parts of the filesystem path needed to disambiguate buffers of
identically named files, as opposed to prefixing refile targets with
the full filesystem path.

lisp/org.el

@@ -2553,13 +2553,16 @@ When the value is `file', also include the file name (without directory)
 into the path.  In this case, you can also stop the completion after
 the file name, to get entries inserted as top level in the file.
 
-When `full-file-path', include the full file path."
+When `full-file-path', include the full file path.
+
+When `buffer-name', use the buffer name."
   :group 'org-refile
   :type '(choice
 	  (const :tag "Not" nil)
 	  (const :tag "Yes" t)
 	  (const :tag "Start with file name" file)
-	  (const :tag "Start with full file path" full-file-path)))
+	  (const :tag "Start with full file path" full-file-path)
+	  (const :tag "Start with buffer name" buffer-name)))
 
 (defcustom org-outline-path-complete-in-steps t
   "Non-nil means complete the outline path in hierarchical steps.
@@ -11557,6 +11560,8 @@ order.")
 	       (setq f (and f (expand-file-name f)))
 	       (when (eq org-refile-use-outline-path 'file)
 		 (push (list (file-name-nondirectory f) f nil nil) tgs))
+	       (when (eq org-refile-use-outline-path 'buffer-name)
+		 (push (list (buffer-name (buffer-base-buffer)) f nil nil) tgs))
 	       (org-with-wide-buffer
 		(goto-char (point-min))
 		(setq org-outline-path-cache nil)
@@ -11585,6 +11590,9 @@ order.")
 				   (`full-file-path
 				    (list (buffer-file-name
 					   (buffer-base-buffer))))
+				   (`buffer-name
+				    (list (buffer-name
+					   (buffer-base-buffer))))
 				   (_ nil))
 				 (mapcar #'org-protect-slash
 					 (org-get-outline-path t t)))