Add --temp and --stage=all options to checkout-index.

Add --temp and --stage=all options to checkout-index.

Sometimes it is convient for a Porcelain to be able to checkout all
unmerged files in all stages so that an external merge tool can be
executed by the Porcelain or the end-user.  Using git-unpack-file
on each stage individually incurs a rather high penalty due to the
need to fork for each file version obtained.  git-checkout-index -a
--stage=all will now do the same thing, but faster.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>