bug#8927: 24.0.50; Flaw in C-x v u

[Chong Yidong <cyd@stupidchicken.com>]

Richard Stallman <rms@gnu.org> writes:

>     Anyway, that's why it kills your *vc-diff* buffer - because it assumes
>     it has just created it. The fix for your problem would presumably be for
>     vc-revert to only kill the buffer if it did not exist beforehand.
>
> C-x v u should preserve the previous contents of *vc-diff*.

I've checked in a fix to avoid clobbering an existing vc-diff buffer
when vc-revert shows a diff temporarily.

I agree with Glenn, though, that showing a vc-diff buffer for this
operation is not a good idea.  Apart from the problems he mentioned, the
diff is not useful anyway, since there is no way to scroll the window
during the yes-nor-no prompt.  I added a new option vc-revert-show-diff
to control whether vc-revert should show a diff, and made it default to
nil.