bug#488: VC error: mark-resolved not implemented for CVS

[Chong Yidong]

> Suppose a CVS file is not up to date.  Set debug-on-error to t.  Make
> an incompatible edit to the file, then do
>
> C-x v v
>
> Since the file is not up-to-date, this signals an error, as expected.
>
> C-x v +
>
> to pull in the conflicting changes.  After resolving the changes,
>
> C-x v v 
>
> Debugger entered--Lisp error: (error "Sorry, mark-resolved is not
> implemented for CVS")

I noticed that doing C-x v + twice causes the file to be marked as
up-to-date.  So maybe C-x v v, when called on a CVS file marked as
conflicted, should try to run `vc-update' again.

Any thoughts?