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>

git-grep.sh

@@ -39,5 +39,9 @@ while : ; do
 	esac
 	shift
 done
+[ "$pattern" ] || {
+	echo >&2 "usage: 'git grep <pattern> [pathspec*]'"
+	exit 1
+}
 git-ls-files -z "${git_flags[@]}" "$@" |
 	xargs -0 grep "${flags[@]}" -e "$pattern"