Transfer FILE to another version control system NEW-BACKEND.
If NEW-BACKEND has a higher precedence than FILE's current backend
(i.e. it comes earlier in `vc-handled-backends'), then register FILE in
NEW-BACKEND, using the revision number from the current backend as the
base level. If NEW-BACKEND has a lower precedence than the current
backend, then commit all changes that were made under the current
backend to NEW-BACKEND, and unregister FILE from the current backend.
(If FILE is not yet registered under NEW-BACKEND, register it.)
(fn FILE NEW-BACKEND)