* vc.el (vc-mark-resolved): Add `backend' argument.

* vc.el (vc-mark-resolved): Add `backend' argument.
(vc-next-action): Pass it the backend.
(vc-next-action, vc-checkout, vc-mark-resolved, vc-version-diff)
(vc-merge, vc-rollback, vc-update, vc-transfer-file, vc-delete-file)
(vc-default-comment-history, vc-default-create-snapshot)
(vc-default-retrieve-snapshot, vc-default-revert, vc-annotate)
(vc-annotate-revision-previous-to-line)
(vc-annotate-show-diff-revision-at-line, vc-annotate-warp-revision):
* vc-svn.el (vc-svn-checkout):
* vc-mcvs.el (vc-mcvs-checkout):
* vc-hooks.el (vc-state, vc-default-workfile-unchanged-p)
(vc-working-revision, vc-before-save, vc-mode-line):
Prefer vc-call-backend to vc-call so as not to recompute the backend.