Revert the disputed VC change and update the tests

Revert the disputed VC change and update the tests

* lisp/vc/vc-hooks.el (vc-working-revision):
Remove the previous change.
(vc-state): Same.  And update the old, incorrect comment about
unregistered files
(http://lists.gnu.org/archive/html/emacs-devel/2016-04/msg00526.html).

* test/lisp/vc/vc-tests.el (vc-test--state): Remove the check
calling `vc-state' on default-directory (VC state is undefined
for directories).  Check that `vc-state' returns nil where it
returned `unregistered' before.  Remove all checks comparing
invocations with the backend passed in explictly and without.
(vc-test--working-revision): Remove all checks comparing
invocations with the backend passed in explictly and without.
Update comments, and add a new one.