Change scaling algorithm for vc-annotate.

Change scaling algorithm for vc-annotate.
From JD Smith <jdsmith@astro.cornell.edu>.
(vc-annotate-display-default): Accept colormap scaling
ratio (now deprecated).
(vc-annotate-display-autoscale): Added.
(vc-annotate-add-menu): New autoscaling menu options "Span to
Oldest" and "Span Oldest->Newest".  Easymenu support added for
toggle menus driven by customize variable
`vc-annotate-display-mode'.
(vc-annotate-display-select): Added.
(vc-annotate): Changed temp-buffer-show-function to
`vc-annotate-display-select'.
(vc-annotate-display): Removed arguments BUFFER and BACKEND.
Added argument OFFSET.  Instead of backend function, calls now
generic `vc-annotate-difference'.
(vc-annotate-difference): Added as generic function instead of
backend-specific function.  No longer takes argument POINT, but
instead accepts a time OFFSET.
(vc-default-annotate-current-time): Added.