bug#43464: 28.0.50; vc: Error calling vc-revert for repo root

[Dmitry Gutov]

On 17.09.2020 10:29, Andrii Kolomoiets wrote:
> Debugger entered--Lisp error: (file-missing "Cannot open load file" "No such file or 
> directory" "vc-nil")
>    require(vc-nil)
>    vc-find-backend-function(nil make-version-backups-p)
>    vc-call-backend(nil make-version-backups-p "/private/tmp/gittest/")
>    vc-version-backup-file("/private/tmp/gittest/")
>    vc-revert-file("/private/tmp/gittest/")
>    vc-revert()
>    funcall-interactively(vc-revert)
>    call-interactively(vc-revert nil nil)
>    command-execute(vc-revert)
>
> At least for `hg` and `git` backends.  Maybe because `vc-registered` for
> repo root is nil.

I don't think we support vc-revrt on directories, repo root or not.