C-x v u for added files in vc-git

[Dan Nicolaescu]

In a git tree doing:

C-x C-f BLAH_BLAH RET
RET
C-x v v

will put the BLAH_BLAH in the 'added state

Doing a C-x v u
after that should return it to the 'unregistered state, but that does
not happen the file stays in the 'added state.

This is because vc-git-revert runs:
(vc-git-command nil 0 file "checkout" "HEAD")

What should vc-git-revert do that it works in the above case too?