API simplification: remove vc-workfile-unchanged-p from pubic methods.

* vc/vc.el, vc-hooks.el, and all backends: API simplification;
vc-workfile-unchanged-p is no longer a public method (but the RCS and
SCCS back ends retain it as a private method used in state
computation). This method was redundant with vc-state and usually
implemented as a trivial call to same. Fixes the failure mode
described in bug#694.

lisp/vc/vc-hg.el

@@ -51,7 +51,6 @@
 ;; * working-revision (file)                   OK
 ;; - latest-on-branch-p (file)                 ??
 ;; * checkout-model (files)                    OK
-;; - workfile-unchanged-p (file)               OK
 ;; - mode-line-string (file)                   NOT NEEDED
 ;; STATE-CHANGING FUNCTIONS
 ;; * register (files &optional rev comment)    OK
@@ -530,10 +529,6 @@ REV is the revision to check out into WORKFILE."
     (message "There are unresolved conflicts in this file")))
 
 
-;; Modeled after the similar function in vc-bzr.el
-(defun vc-hg-workfile-unchanged-p (file)
-  (eq 'up-to-date (vc-hg-state file)))
-
 ;; Modeled after the similar function in vc-bzr.el
 (defun vc-hg-revert (file &optional contents-done)
   (unless contents-done