patch 9.0.0086: tabline is not redrawn when entering command line

Problem:    Tabline is not redrawn when entering command line.
Solution:   Set "redraw_tabline". (closes #10771)
diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index 1994a8c..e1ee633 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -3173,4 +3173,20 @@
   silent! norm Q00000000000000     000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000                                                                                                                                                                                                                        
 endfunc
 
+func Test_cmdline_redraw_tabline()
+  CheckRunVimInTerminal
+
+  let lines =<< trim END
+      set showtabline=2
+      autocmd CmdlineEnter * set tabline=foo
+  END
+  call writefile(lines, 'Xcmdline_redraw_tabline')
+  let buf = RunVimInTerminal('-S Xcmdline_redraw_tabline', #{rows: 6})
+  call term_sendkeys(buf, ':')
+  call WaitForAssert({-> assert_match('^foo', term_getline(buf, 1))})
+
+  call StopVimInTerminal(buf)
+  call delete('Xcmdline_redraw_tabline')
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab