make 'git add' a first class user friendly interface to the index

This brings the power of the index up front using a proper mental model
without talking about the index at all. See for example how all the
technical discussion has been evacuated from the git-add man page.

   Any content to be committed must be added together.  Whether that
   content comes from new files or modified files doesn't matter.  You
   just need to "add" it, either with git-add, or by providing
   git-commit with -a (for already known files only of course).

No need for a separate command to distinguish new vs modified files
please. That would only screw the mental model everybody should have
when using GIT.

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