(vc-next-action-on-file): Update mode line only if file is visited.

(vc-next-action-on-file): Update mode line only if file is visited.
(vc-start-entry): New argument initial-contents.  Don't visit the file
if it isn't already visited.  Brought documentation up-to-date.
(vc-next-action, vc-register): Updated calls to vc-start-entry.
(vc-checkin): New optional arg initial-contents, which is passed to
vc-start-entry.
(vc-finish-logentry): Make sure to bury log buffer only if there
really is one.  Call `vc-resynch-buffer' on log-file, not
buffer-file-name.
(vc-default-comment-history, vc-default-wash-log): New functions.
(vc-index-of): Removed.
(vc-transfer-file): Make do without the above.
(vc-default-receive-file): Call comment-history unconditionally.  Pass
the resulting string to vc-checkin, instead of inserting it into the
comment ring.