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.
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