Do "git add" as a builtin

Do "git add" as a builtin

First try. Let's see how well this works.

In many ways, the hard parts of "git commit" are not so different from
this, and a builtin commit would share a lot of the code, I think.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>