files-backend: prefer "0" for write_in_full() error check

Commit 06f46f237a (avoid "write_in_full(fd, buf, len) != len" pattern, 2017-09-13) converted this callsite from: write_in_full(...) != 1 to write_in_full(...) < 0 But during the conflict resolution in c50424a6f0 (Merge branch 'jk/write-in-full-fix', 2017-09-25), this morphed into write_in_full(...) < 1 This behaves as we want, but we prefer to avoid modeling the "less than length" error-check which can be subtly buggy, as shown in efacf609c8 (config: avoid "write_in_full(fd, buf, len) < len" pattern, 2017-09-13). Signed-off-by: Jeff King <peff@peff.net> Reviewed-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>