git-blame.el: pick a set of random colors for each git-blame turn

git-blame.el: pick a set of random colors for each git-blame turn

I thought it would be cool to have different set of colors for each
git-blame-mode. Function `git-blame-new-commit' does this for us
picking when possible, a random colors based on the set we build on
startup. When it fails, `git-blame-ancient-color' will be used. We
also take care not to use the same color more than once (thank you
David Kågedal, really).

* Prevent (future possible) namespace clash by renaming `color-scale'
into `git-blame-color-scale'. Definition has been changed to be more
in the "lisp" way (thanks for help to #emacs). Also added a small
description of what it does.

* Added docstrings at some point and instructed defvar when a variable
was candidate to customisation by users.

* Added missing defvar to silent byte-compilers (git-blame-file,
git-blame-current)

* Do not require 'cl at startup

* Added more informations on compatibility

Signed-off-by: Xavier Maillard <zedek@gnu.org>
Acked-by: David Kågedal <davidk@lysator.liu.se>
Signed-off-by: Junio C Hamano <junkio@cox.net>