Problem: 8defe1a declared the "more" window the most convenient place to
route messages to if it is already open for msg.pos == 'cmd'.
In usage, this doesn't appear to be the case. Appending messages
as added in that commit is still useful, but should only be done
for messages that spill 'cmdheight'.
Solution: Only append messages exceeding 'cmdheight' to the more window.
To do this, instead of immediately writing to the more buffer,
write to the cmd buffer and calculate its height. Then copy the
text and its highlights to the more buffer.
76f76fb083
