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

Commit 06f46f23 (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 c50424a6 (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 efacf609 (config: avoid "write_in_full(fd, buf,
len) < len" pattern, 2017-09-13).
Signed-off-by: NJeff King <peff@peff.net>
Reviewed-by: NJonathan Nieder <jrnieder@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

refs/files-backend.c

@@ -3007,7 +3007,7 @@ static int files_reflog_expire(struct ref_store *ref_store,
 		} else if (update &&
 			   (write_in_full(get_lock_file_fd(&lock->lk),
 				oid_to_hex(&cb.last_kept_oid), GIT_SHA1_HEXSZ) < 0 ||
-			    write_str_in_full(get_lock_file_fd(&lock->lk), "\n") < 1 ||
+			    write_str_in_full(get_lock_file_fd(&lock->lk), "\n") < 0 ||
 			    close_ref_gently(lock) < 0)) {
 			status |= error("couldn't write %s",
 					get_lock_file_path(&lock->lk));