update-hook: abort early if the project description is unset

update-hook: abort early if the project description is unset

It was annoying to always have the first email from a project be from
the "Unnamed repository; edit this file to name it for gitweb project";
just because it's so easy to forget to set it.

This patch checks to see if the description file is still default (or
empty) and aborts if so - allowing you to fix the problem before sending
out silly looking emails to every developer.

Signed-off-by: Andy Parkins <andyparkins@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>