Disallow empty pattern in "git grep"

Disallow empty pattern in "git grep"

For some reason I've done a "git grep" twice with no pattern, which is
really irritating, since it just grep everything. If I actually wanted
that, I could do "git grep ^" or something.

So add a "usage" message if the pattern is empty.

Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>