default file for diff-backup

[ams@gnu.org (Alfred M. Szmidt)]

This makes diff-backup supply a default file name, making the
default behaviour a bit more useful.

*** lisp/vc/diff.el.~1~
--- lisp/vc/diff.el
***************
*** 187,194 ****
  If this file is a backup, diff it with its original.
  The backup file is the first file given to `diff'.
  With prefix arg, prompt for diff switches."
!   (interactive (list (read-file-name "Diff (file with backup): ")
! 		     (diff-switches)))
    (let (bak ori)
      (if (backup-file-name-p file)
  	(setq bak file
--- 187,201 ----
  If this file is a backup, diff it with its original.
  The backup file is the first file given to `diff'.
  With prefix arg, prompt for diff switches."
!   (interactive
!    (list
!     (if (and buffer-file-name (file-exists-p buffer-file-name))
! 	(read-file-name
! 	 (concat "Diff file with backup (default "
! 		 (file-name-nondirectory buffer-file-name) "): ")
! 	 nil buffer-file-name t)
!       (read-file-name "Diff file with backup: " nil nil t))
!     (diff-switches)))
    (let (bak ori)
      (if (backup-file-name-p file)
  	(setq bak file

Diff finished.  Fri Jun 20 14:06:26 2014