Add git-clean command

This command removes untracked files from the working tree.  This
implementation is based on cg-clean with some simplifications.  The
documentation is included.

[jc: with trivial documentation fix, noticed by Jakub Narebski]

Signed-off-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>