(vc-context-matches-p): New function.

(vc-context-matches-p): New function.
(vc-restore-buffer-context): Restore point and mark only if they don't
match the context.
(vc-revert-buffer1, vc-clear-headers): Use save-excursion to relocate
point and mark, and vc-restore-buffer-context as a backup.
(vc-resynch-buffer): When operating on the current buffer, don't use
save-excursion, because that would undo the effects of the above
functions.
(vc-resynch-window): Deleted code that removed vc-find-file-hook
temporarily.  This was unnecessary, because find-file-hooks are not
called when the buffer is reverted.
(vc-register): Added comment for prev change.