git-clone: print an error message when trying to clone empty repo

git-clone: print an error message when trying to clone empty repo

Previously, cloning an empty repository looked like this:

$ (mkdir parent && cd parent && git --bare init)
$ git-clone parent child
Initialized empty Git repository in /home/peff/clone/child/.git/
$ cd child
-bash: cd: child: No such file or directory
$ echo 'wtf?' | mail git@vger.kernel.org

Now we at least report that the clone was not successful.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>