use 'init' instead of 'init-db' for shipped docs and tools

use 'init' instead of 'init-db' for shipped docs and tools

While 'init-db' still is and probably will always remain a valid git
command for obvious backward compatibility reasons, it would be a good
idea to move shipped tools and docs to using 'init' instead.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>