sparse-checkout: check commit_lock_file when writing patterns

When writing a new "sparse-checkout" file, we do the usual strategy of writing to a lockfile and committing it into place. But we don't check the outcome of commit_lock_file(). Failing there would prevent us from writing a bogus file (good), but we would ignore the error and return a successful exit code (bad). Fix this by calling die(). Note that we need to keep the sparse_filename variable valid for longer, since the filename stored in the lock_file struct will be dropped when we run commit_lock_file(). Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>