git-init-db: create "pack" subdirectory under objects

Don't make the user have to mkdir it just because he's excited about the
new object pack functionality, do it for him.

init-db.c

@@ -87,5 +87,7 @@ int main(int argc, char **argv)
 		sprintf(path+len, "/%02x", i);
 		safe_create_dir(path);
 	}
+	strcpy(path+len, "/pack");
+	safe_create_dir(path);
 	return 0;
 }