sparse-checkout: free sparse_filename after use

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>