notes-merge: drop dead zero-write code

We call write_in_full() with a size that we know is greater
than zero. The return value can never be zero, then, since
write_in_full() converts such a failed write() into ENOSPC
and returns -1.  We can just drop this branch of the error
handling entirely.

Suggested-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

notes-merge.c

@@ -308,8 +308,6 @@ static void write_buf_to_worktree(const struct object_id *obj,
 			if (errno == EPIPE)
 				break;
 			die_errno("notes-merge");
-		} else if (!ret) {
-			die("notes-merge: disk full?");
 		}
 		size -= ret;
 		buf += ret;