sparse-checkout: refactor temporary sparse_checkout_patterns

In update_working_directory(), we take in a pattern_list, attach it to the repository index by assigning it to index->sparse_checkout_patterns, and then call unpack_trees. Afterwards, we remove it by setting index->sparse_checkout_patterns back to NULL. But there are two possible leaks here: 1. If the index already had a populated sparse_checkout_patterns, we've obliterated it. We can fix this by saving and restoring it, rather than always setting it back to NULL. 2. We may call the function with a NULL pattern_list, expecting it to use the on-disk sparse file. In that case, the index routines will lazy-load the sparse patterns automatically. But now at the end of the function when we restore the patterns, we'll leak those lazy-loaded ones! We can fix this by freeing the pattern list before overwriting its pointer whenever it does not match what was passed in (in practice this should only happen when the passed-in list is NULL, but this is erring on the defensive side). Together these remove 48 indirect leaks found in t1091. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>