add cmdline mode to modechange of RPC and tests

use set_cursor_shape_bar for cmdline mode

fix test of screen_basic_spec.lua & screen.lua

comment fix

test/functional/ui/screen.lua

@@ -374,7 +374,8 @@ function Screen:_handle_mouse_off()
 end
 
 function Screen:_handle_mode_change(mode)
-  assert(mode == 'insert' or mode == 'replace' or mode == 'normal')
+  assert(mode == 'insert' or mode == 'replace'
+        or mode == 'normal' or mode == 'cmdline')
   self.mode = mode
 end