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.
So do

C-x v +

to pull in the conflicting changes.  This adds the usual <<< and >>>
lines to the file.  After resolving the changes, try to check in the
file:

C-x v v 

The result:

Debugger entered--Lisp error: (error "Sorry, mark-resolved is not implemented 
for CVS")
  signal(error ("Sorry, mark-resolved is not implemented for CVS"))
  error("Sorry, %s is not implemented for %s" mark-resolved CVS)
  vc-call-backend(CVS mark-resolved ("/home/cyd/trunk/src/ChangeLog"))
  vc-mark-resolved(CVS ("/home/cyd/trunk/src/ChangeLog"))
  vc-next-action(nil)
  call-interactively(vc-next-action nil nil)