patch 8.0.0964: channel write buffer does not work with poll()

Problem:    Channel write buffer does not work with poll().
Solution:   Use the same mechanism as with select().
diff --git a/src/channel.c b/src/channel.c
index 9a14e1e..79cca79 100644
--- a/src/channel.c
+++ b/src/channel.c
@@ -3018,7 +3018,9 @@
     {
 	chanpart_T  *in_part = &ch->ch_part[PART_IN];
 
-	if (in_part->ch_fd != INVALID_FD && in_part->ch_bufref.br_buf != NULL)
+	if (in_part->ch_fd != INVALID_FD
+		&& (in_part->ch_bufref.br_buf != NULL
+		    || in_part->ch_writeque.wq_next != NULL))
 	{
 	    in_part->ch_poll_idx = nfd;
 	    fds[nfd].fd = in_part->ch_fd;
@@ -3946,13 +3948,7 @@
 	idx = in_part->ch_poll_idx;
 	if (ret > 0 && idx != -1 && (fds[idx].revents & POLLOUT))
 	{
-	    if (in_part->ch_buf_append)
-	    {
-		if (in_part->ch_bufref.br_buf != NULL)
-		    channel_write_new_lines(in_part->ch_bufref.br_buf);
-	    }
-	    else
-		channel_write_in(channel);
+	    channel_write_input(channel);
 	    --ret;
 	}
     }