git grep: Add "-z/--null" option as in GNU's grep.

git grep: Add "-z/--null" option as in GNU's grep.

Here's a trivial patch that adds "-z" and "--null" options to "git
grep". It was discussed on the mailing-list that git's "-z"
convention should be used instead of GNU grep's "-Z".
So things like 'git grep -l -z "$FOO" | xargs -0 sed -i "s/$FOO/$BOO/"'
do work now.

Signed-off-by: Raphael Zimmerer <killekulla@rdrz.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>