bug#32225: 27.0.50; [PATCH] avoid calling vc backend if vc-display-status is nil

[Eli Zaretskii]

> From: Andy Moreton <andrewjmoreton@gmail.com>
> Date: Fri, 20 Jul 2018 17:45:09 +0100
> 
> If the user sets `vc-display-status' to nil then the VC backend is still
> called to get a revision number, even though it will not be displayed.
> 
> The following patch avoids this unneeded work.
> 
> 
> Do not call VC backend to update mode line if vc-display-status is nil
> 
> 2018-07-20  Andy Moreton  <andrewjmoreton@gmail.com>
> 
>       * lisp/vc/vc-hooks.el (vc-mode-line): Avoid calling VC backend if
>       vc-display-status is nil.

LGTM, please push to the master branch.

Thanks.