test-delta: close output descriptor after use

After we write to the output file, the program exits. This naturally closes the descriptor. But we should do an explicit close for two reasons: 1. It's possible to hit an error on close(), which we should detect and report via our exit code. 2. Leaking descriptors is a bad practice in general. Even if it isn't meaningful here, it sets a bad example. It is tempting to write: if (write_in_full(fd, ...) < 0 || close(fd) < 0) die_errno(...); But that pattern contains a subtle problem that has resulted in descriptor leaks before. If write_in_full() fails, we'll short-circuit and never call close(), leaking the descriptor. That's not a problem here, since our error path dies instead of returning up the stack. But since we're trying to set a good example, let's write it out as two separate conditions. As a bonus, that lets us produce a slightly more specific error message. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>