update-index: --stdin and -z

update-index: --stdin and -z

The new option --stdin reads list of paths to be updated from the
standard input.  As usual, -z means the paths are terminated with NUL
characters, as opposed to LF without that option.

This is useful to use git-diff-files -z and git-ls-files -z when the
platform xargs does not support -0 option, and obviously saves one
process even when xargs can take -0.

Signed-off-by: Junio C Hamano <junkio@cox.net>