vc-allow-async-revert

vc-allow-async-revert is a variable defined in `vc.el'.
Its value is

nil

Documentation:
Specifies whether the diff during C-x v u may be asynchronous.
Enabling this option means that you can confirm a revert operation even
if the local changes in the file have not been found and displayed yet.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 22.1 of Emacs.