New tutorial

New tutorial

The current Documentation/tutorial.txt concentrates on the lower-level
git interfaces.  So it's useful to people developing alternative
porcelains, to advanced users, etc., but not so much to beginning users.

I think it makes sense for the main tutorial to address those
beginnning users, so with this patch I'm proposing that we move
Documentation/tutorial.txt to Documentation/core-tutorial.txt and
replace it by a new tutorial.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Junio C Hamano <junkio@cox.net>