(vc-arch-diff): Handle the special case where `newvers' is equivalent to nil.

(vc-arch-diff): Handle the special case where `newvers' is equivalent to nil.
(vc-arch-diff3-rej-p): Be a bit more flexible in what we accept.
(vc-arch-mode-line-string): Accept `added' state.
(vc-arch-state): Use inode-sigs if available.
(vc-arch-add-tagline): Rename from vc-arch-add-tag.
Copy&delete existing id file if any.  Fallback if uuidgen is absent.
(vc-arch-tagline-re): New var.
(vc-arch-file-source-p, vc-arch-file-id, vc-arch-tagging-method): New funs.
(vc-arch-find-file-not-found-hook, vc-arch-register): New backend ops.
(vc-arch-registered): Try our best guess using vc-arch-file-source-p.