Remove `diff-tree' operation, now subsumed by `diff'.

Remove `diff-tree' operation, now subsumed by `diff'.
Also `revision-completion-table' now takes a list of files.
(vc-deduce-fileset): Remove unused var `regexp'.
Only obey allow-directory-wildcard in dired buffers.
(vc-default-diff-tree): Remove.
(vc-diff-added-files): New var.
(vc-diff-internal): Use it.  Remove arg `backend'.  Update callers.
(vc-version-diff): Revert from `vc-history-diff' to the original name.
Remove the `backend' arg.
(vc-contains-version-controlled-file): Remove.
(vc-diff): Bring it closer to the version in Emacs-22.
(vc-revert): Fix typo in let-binding.
(vc-default-unregister): Remove.
(vc-dired-buffers-for-dir): Remove N^2 behavior.