If you re-run M-x vc-diff from a *vc-diff* buffer, it doesn't ask you to
save the buffer being diffed the way that it does when operating from
that buffer itself

    C-x C-f /some/file/under/vc/control
    C-x v =
    C-x o
    # edit the file's buffer a bit
    C-x o           # back to the *vc-diff*
    C-x v =

    => doesn't ask about saving the edits

I wonder also if vc-diff might offer to save the whole of its "fileset",
something like below.  Not that I'm not smart enough to diff more than
one file at a time :-)

Maybe `vc-update' could share this `vc-buffer-sync-fileset' too.
I thought to use find-buffer-visiting instead of the way vc-update has
`member' in its save-some-buffers, just in case there's symlinks making
names look different.

2010-03-25  Kevin Ryde  <user42@zip.com.au>

	* vc.el (vc-buffer-sync-fileset): New function.
	(vc-diff): Use it to save all relevant file buffers no matter
	where run (not just the current buffer).