patch 8.2.0131: command line is not cleared when switching tabs

Problem:    Command line is not cleared when switching tabs and the command
            line height differs.
Solution:   Set the "clear_cmdline" flag when needed. (Naruhiko Nishino,
            closes #5495)
diff --git a/src/testdir/dumps/Test_cmdlineclear_tabenter.dump b/src/testdir/dumps/Test_cmdlineclear_tabenter.dump
new file mode 100644
index 0000000..27dcca5
--- /dev/null
+++ b/src/testdir/dumps/Test_cmdlineclear_tabenter.dump
@@ -0,0 +1,10 @@
+| +8#0000001#e0e0e08|+| |[|N|o| |N|a|m|e|]| | +2#0000000#ffffff0|[|N|o| |N|a|m|e|]| | +1&&@49|X+8#0000001#e0e0e08
+> +0#0000000#ffffff0@74
+|~+0#4040ff13&| @73
+|~| @73
+|~| @73
+|~| @73
+|~| @73
+| +0#0000000&@74
+@75
+@57|0|,|0|-|1| @8|A|l@1| 
diff --git a/src/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index 6dc6c89..36067d1 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -905,4 +905,22 @@
   delfunc CmdWinType
 endfunc
 
+func Test_cmdlineclear_tabenter()
+  CheckScreendump
+
+  let lines =<< trim [SCRIPT]
+    call setline(1, range(30))
+  [SCRIPT]
+
+  call writefile(lines, 'XtestCmdlineClearTabenter')
+  let buf = RunVimInTerminal('-S XtestCmdlineClearTabenter', #{rows: 10})
+  call term_wait(buf, 50)
+  " in one tab make the command line higher with CTRL-W -
+  call term_sendkeys(buf, ":tabnew\<cr>\<C-w>-\<C-w>-gtgt")
+  call VerifyScreenDump(buf, 'Test_cmdlineclear_tabenter', {})
+
+  call StopVimInTerminal(buf)
+  call delete('XtestCmdlineClearTabenter')
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab