[PATCH 2/2] core-git documentation update

Reformat core-git.txt to asciidoc format.
Includes split-docs.pl to create individual txt, html and man pages.

<JC> Editorial note.  I've updated to add git-diff-cache -m and
git-update-cache --replace description on top of the version
David posted to the GIT list and got his OK.

Signed-off-by: David Greaves <david@dgreaves.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>