vim-patch:8.2.2686: status line is not updated when going to cmdline mode

Problem:    Status line is not updated when going to cmdline mode.
Solution:   Redraw status lines if 'statusline' is set and going to status
            line mode. (based on patch from Justin M. Keyes et al.,
            closes vim/vim#8044)
ce0b75711a

src/nvim/ex_getln.c

@@ -775,9 +775,9 @@ static uint8_t *command_line_enter(int firstc, long count, int indent)
     redrawcmd();
   }
 
-  // redraw the statusline for statuslines that display the current mode
-  // using the mode() function.
-  if (!cmd_silent && msg_scrolled == 0) {
+  // Redraw the statusline in case it uses the current mode using the mode()
+  // function.
+  if (!cmd_silent && msg_scrolled == 0 && *p_stl != NUL) {
     curwin->w_redr_status = true;
     redraw_statuslines();
   }

src/nvim/testdir/test_statusline.vim

@@ -440,6 +440,27 @@ func Test_statusline_removed_group()
   call delete('XTest_statusline')
 endfunc
 
+func Test_statusline_using_mode()
+  CheckScreendump
+
+  let lines =<< trim END
+    set laststatus=2
+    let &statusline = '-%{mode()}-'
+  END
+  call writefile(lines, 'XTest_statusline')
+
+  let buf = RunVimInTerminal('-S XTest_statusline', {'rows': 5, 'cols': 50})
+  call VerifyScreenDump(buf, 'Test_statusline_mode_1', {})
+
+  call term_sendkeys(buf, ":")
+  call VerifyScreenDump(buf, 'Test_statusline_mode_2', {})
+
+  " clean up
+  call term_sendkeys(buf, "\<CR>")
+  call StopVimInTerminal(buf)
+  call delete('XTest_statusline')
+endfunc
+
 func Test_statusline_after_split_vsplit()
   only