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>

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