Teach git-checkout-index to read filenames from stdin.

Teach git-checkout-index to read filenames from stdin.

Since git-checkout-index is often used from scripts which
may have a stream of filenames they wish to checkout it is
more convenient to use --stdin than xargs.  On platforms
where fork performance is currently sub-optimal and
the length of a command line is limited (*cough* Cygwin
*cough*) running a single git-checkout-index process for
a large number of files beats spawning it multiple times
from xargs.

File names are still accepted on the command line if
--stdin is not supplied.  Nothing is performed if no files
are supplied on the command line or by stdin.

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