vc-annotate error

[JD Smith <jdsmith@as.arizona.edu>]

vc-annotate is giving me a similar error as was reported earlier for
vc.el revision 1.408:

 (file-error "Cannot open load file" "vc-nil")

This occurs because I have specified a default annotate mode of
'fullscale, and the `vc-annotate-display-autoscale' function still
relies on the variable `vc-annotate-backend' to call the appropriate
backend time functions I wrote years ago.  This was formerly a global
variable, but Stefan's changes to `vc-annotate' circa 1.408 made it buffer
local:

    (with-current-buffer temp-buffer-name
      (set (make-local-variable 'vc-annotate-backend) (vc-backend file))
      (set (make-local-variable 'vc-annotate-parent-file) file)
      (set (make-local-variable 'vc-annotate-parent-rev) rev)
      (set (make-local-variable 'vc-annotate-parent-display-mode)
	   display-mode))

For whatever reason, these are not currently being set in the annotate
output buffer, which breaks the auto-scaling annotation display, since
it doesn't know the backend to use.  I haven't figured out how the
buffer-local variables are being lost.  Other annotate display modes
don't have backend-specific functions, so they continue to work.

By the way, should we make 'fullscale the default display mode?
Rather than scaling colors based on a fixed time window (1 year I
believe is the current default), it auto-scales to ensure the oldest
and newest colors both are used, to match the dynamic range of
annotation dates in the file to the available number of display
colors.  Currently, files less than a year old will be displayed
entirely in one color.

JD