* vc.el (vc-default-previous-version): Doc enhancement.

* vc.el (vc-default-previous-version): Doc enhancement.
(vc-default-next-version): New function.
(vc-print-log): New arg FOCUS-REV.
(vc-annotate-mode):  Derives from view-mode now.
(vc-annotate):  New args REVISION, DISPLAY-MODE.
(vc-annotate-prev-version): New function.
(vc-annotate-prev-version): New function.
(vc-annotate-next-version): New function.
(vc-annotate-workfile-version): New function.
(vc-annotate-extract-revision-at-line): New function.
(vc-annotate-revision-at-line): New function.
(vc-annotate-revision-previous-to-line): New function.
(vc-annotate-show-log-revision-at-line): New function.
(vc-annotate-show-diff-revision-at-line): New function.
(vc-current-line): New function.
(vc-annotate-warp-version): New function.