(vc-revert-buffer1): Split part of the function into vc-buffer-context

(vc-revert-buffer1): Split part of the function into vc-buffer-context
and vc-restore-buffer-context, so we can use it also in other
circumstances.
(vc-buffer-context, vc-restore-buffer-context): New functions.
(vc-clear-headers): New function, uses the above.
(vc-cancel-version): When `norevert', locks the most recent remaining
version.  Also, refuse to work on anything but the latest version of
a branch.  Removed the check whether the version is the user's,
because that is difficult to decide, now that multiple branches are
possible.
(vc-latest-on-branch-p): New function.
(vc-head-version): New access function to the already existing
property.
(vc-trunk-p, vc-branch-part): Functions moved before first use.