bug#15569: 24.3.50; vc-ediff with prefix argument could be slightly better

[João Távora]

Hello maintainters,

I noticed that, in a git repository, using C-u M-x vc-ediff on a single
file (a very useful command) could use a slight improvement.

You see, using C-u prompts the users for two revisions. I mostly choose
to use an ancient revision for A and the "current revision" for B. And
thats the default, which is fine.

But there is a catch. If I've not yet touched the file I'm vc-ediff'ing,
then buffer B will not be the actual buffer I'm working on, but a
read-only copy of it, which is not good for copying content over from A
to B using the ediff keys "A" and "B".

The workaround I and my colleagues currently use is to add some random
change to the file to make sure vc-ediff suggests "current source" as
the revision for B. But this is kinda akward.

I've found that the following patch fixes this behaviour, and still
makes sense to me.

João

z:/Vendor/cygwin/bin/diff.exe -c "--label" 
"z:/Vendor/emacs-24.3.5/share/emacs/24.3.50/lisp/vc/vc.el" "--label" "#<buffer 
vc.el>" "z:/Vendor/emacs-24.3.5/share/emacs/24.3.50/lisp/vc/vc.el" 
"c:/temp/buffer-content-5840WVj"
*** z:/Vendor/emacs-24.3.5/share/emacs/24.3.50/lisp/vc/vc.el
--- #<buffer vc.el>
***************
*** 1735,1747 ****
       ;; if the file is not up-to-date, use working revision as older revision
       ((not (vc-up-to-date-p first))
        (setq rev1-default (vc-working-revision first)))
!      ;; if the file is not locked, use last and previous revisions as defaults
       (t
        (setq rev1-default (ignore-errors ;If `previous-revision' doesn't work.
                             (vc-call-backend backend 'previous-revision first
                                              (vc-working-revision first))))
!       (when (string= rev1-default "") (setq rev1-default nil))
!       (setq rev2-default (vc-working-revision first))))
      ;; construct argument list
      (let* ((rev1-prompt (if rev1-default
                              (concat "Older revision (default "
--- 1735,1746 ----
       ;; if the file is not up-to-date, use working revision as older revision
       ((not (vc-up-to-date-p first))
        (setq rev1-default (vc-working-revision first)))
!      ;; if the file is not locked, use last revision and current source as 
defaults
       (t
        (setq rev1-default (ignore-errors ;If `previous-revision' doesn't work.
                             (vc-call-backend backend 'previous-revision first
                                              (vc-working-revision first))))
!       (when (string= rev1-default "") (setq rev1-default nil))))
      ;; construct argument list
      (let* ((rev1-prompt (if rev1-default
                              (concat "Older revision (default "

Diff finished.  Wed Oct  9 10:03:04 2013