VIM: search_refresh limits message count to 2 * window.height

[Franz Fellner]

The reason is that StagedRender.is_ready depends on last_render, which
get's set to VIM::Buffer.count() in StagedRender::do_next.
I do not (yet) know what exactly happens, but after the first call to
search refresh last_render never get's less than 2*2*window.height.
That means once you do search_refresh StagedRender never will be ready -
is_ready returns false, so s:show_cursor_moved never will advance the
StagedRender.

I am trying to understand the code, but it's a hard time for me ;)