patch 8.1.2108: cannot close the cmdline window from CmdWinEnter

Problem:    Cannot close the cmdline window from CmdWinEnter. (George Brown)
Solution:   Reset cmdwin_result earlier. (Christian Brabandt, closes #4980)

src/ex_getln.c

@@ -4152,12 +4152,15 @@ open_cmdwin(void)
     invalidate_botline();
     redraw_later(SOME_VALID);
 
-    /* No Ex mode here! */
+    // No Ex mode here!
     exmode_active = 0;
 
     State = NORMAL;
     setmouse();
 
+    // Reset here so it can be set by a CmdWinEnter autocommand.
+    cmdwin_result = 0;
+
     // Trigger CmdwinEnter autocommands.
     trigger_cmd_autocmd(cmdwin_type, EVENT_CMDWINENTER);
     if (restart_edit != 0)	// autocmd with ":startinsert"
@@ -4169,7 +4172,6 @@ open_cmdwin(void)
     /*
      * Call the main loop until <CR> or CTRL-C is typed.
      */
-    cmdwin_result = 0;
     main_loop(TRUE, FALSE);
 
     RedrawingDisabled = i;

src/testdir/test_autocmd.vim

@@ -2259,3 +2259,32 @@ func Test_autocmd_SafeState()
   call StopVimInTerminal(buf)
   call delete('XSafeState')
 endfunc
+
+func Test_autocmd_CmdWinEnter()
+  CheckRunVimInTerminal
+  " There is not cmdwin switch, so
+  " test for cmdline_hist
+  " (both are available with small builds)
+  CheckFeature cmdline_hist
+  let lines =<< trim END
+    let b:dummy_var = 'This is a dummy'
+    autocmd CmdWinEnter * quit
+    let winnr = winnr('$')
+  END
+  let filename='XCmdWinEnter'
+  call writefile(lines, filename)
+  let buf = RunVimInTerminal('-S '.filename, #{rows: 6})
+
+  call term_sendkeys(buf, "q:")
+  call term_wait(buf)
+  call term_sendkeys(buf, ":echo b:dummy_var\<cr>")
+  call WaitForAssert({-> assert_match('^This is a dummy', term_getline(buf, 6))}, 1000)
+  call term_sendkeys(buf, ":echo &buftype\<cr>")
+  call WaitForAssert({-> assert_notmatch('^nofile', term_getline(buf, 6))}, 1000)
+  call term_sendkeys(buf, ":echo winnr\<cr>")
+  call WaitForAssert({-> assert_match('^1', term_getline(buf, 6))}, 1000)
+
+  " clean up
+  call StopVimInTerminal(buf)
+  call delete(filename)
+endfunc

src/version.c

@@ -753,6 +753,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    2108,
 /**/
     2107,
 /**/