* tramp-sh.el (vc-handled-backends, vc-bzr-program)
(vc-git-program, vc-hg-program): Declare.
(tramp-sh-handle-vc-registered): Remove backends where the remote
binary does not exist.
* tramp.el (tramp-methods): Add docstring for `tramp-login-env'
and `tramp-copy-env'.

* tramp-sh.el (tramp-methods) <sudo>: Add `tramp-login-env'.
(tramp-maybe-open-connection): Handle `tramp-login-env'.