sparse-checkout: free sparse_filename after use

We allocate a heap buffer via get_sparse_checkout_filename(). Most calls
remember to free it, but sparse_checkout_init() forgets to, causing a
leak. Ironically, it remembers to do so in the error return paths, but
not in the path that makes it all the way to the function end!

Fixing this clears up 6 leaks from t1091.

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

builtin/sparse-checkout.c

@@ -500,6 +500,8 @@ static int sparse_checkout_init(int argc, const char **argv, const char *prefix)
 		return 0;
 	}
 
+	free(sparse_filename);
+
 	add_pattern("/*", empty_base, 0, &pl, 0);
 	add_pattern("!/*/", empty_base, 0, &pl, 0);
 	pl.use_cone_patterns = init_opts.cone_mode;