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>

git-clone.sh

@@ -297,7 +297,8 @@ yes)
 				      find objects -type f -print | sed -e 1q)
 			# objects directory should not be empty because
 			# we are cloning!
-			test -f "$repo/$sample_file" || exit
+			test -f "$repo/$sample_file" ||
+				die "fatal: cannot clone empty repository"
 			if ln "$repo/$sample_file" "$GIT_DIR/objects/sample" 2>/dev/null
 			then
 				rm -f "$GIT_DIR/objects/sample"