fix(ui): inccomand performance degradation

It was broken since the introduction of the macro.

src/nvim/ex_cmds.c

@@ -4313,17 +4313,21 @@ skip:
 
 #define PUSH_PREVIEW_LINES() \
   do { \
+    if (preview) { \
       linenr_T match_lines = current_match.end.lnum \
                              - current_match.start.lnum +1; \
       if (preview_lines.subresults.size > 0) { \
         linenr_T last = kv_last(preview_lines.subresults).end.lnum; \
         if (last == current_match.start.lnum) { \
           preview_lines.lines_needed += match_lines - 1; \
+        } else { \
+          preview_lines.lines_needed += match_lines; \
         } \
       } else { \
         preview_lines.lines_needed += match_lines; \
       } \
       kv_push(preview_lines.subresults, current_match); \
+    } \
     } while (0)
 
             // Push the match to preview_lines.

test/functional/ui/cmdline_spec.lua

@@ -4,6 +4,7 @@ local clear, feed = helpers.clear, helpers.feed
 local source = helpers.source
 local command = helpers.command
 local assert_alive = helpers.assert_alive
+local uname = helpers.uname
 
 local function new_screen(opt)
   local screen = Screen.new(25, 5)
@@ -824,7 +825,7 @@ describe('cmdline redraw', function()
   end)
 
   it('with <Cmd>', function()
-    if 'openbsd' == helpers.uname() then
+    if string.find(uname(), 'bsd') then
       pending('FIXME #10804')
     end
     command('cmap a <Cmd>call sin(0)<CR>')  -- no-op