(defun vc-version-diff (file rel1 rel2)
"For FILE, report diffs between two stored versions REL1 and REL2 of it.
If FILE is a directory, generate diffs between versions for all registered
- (let ((file (read-file-name (if buffer-file-name
- "File or dir to diff: (default visited file) "
- "File or dir to diff: ")
- default-directory buffer-file-name t))
- (rel1-default nil) (rel2-default nil))
+Interactively, `buffer-file-name' will be used for FILE, unless it's nil, or
+unless a prefix arg is used. In the latter two cases, the user will be queried
+ (let ((file (if (and current-prefix-arg buffer-file-name)
+ "File or dir to diff: (default visited file) "
+ "File or dir to diff: ")
+ default-directory buffer-file-name t)
+ (rel1-default nil) (rel2-default nil))
;; compute default versions based on the file state
;; if it's a directory, don't supply any version defauolt