vim-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 vim/vim#4980)
23324a0b35

src/nvim/ex_getln.c

@@ -6471,12 +6471,15 @@ static int open_cmdwin(void)
   // Save the command line info, can be used recursively.
   save_cmdline(&save_ccline);
 
-  /* 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.
   typestr[0] = (char_u)cmdwin_type;
   typestr[1] = NUL;
@@ -6492,7 +6495,6 @@ static int open_cmdwin(void)
   /*
    * Call the main loop until <CR> or CTRL-C is typed.
    */
-  cmdwin_result = 0;
   normal_enter(true, false);
 
   RedrawingDisabled = i;

src/nvim/testdir/test_autocmd.vim

@@ -1,6 +1,8 @@
 " Tests for autocommands
 
 source shared.vim
+source check.vim
+source term_util.vim
 
 func! s:cleanup_buffers() abort
   for bnr in range(1, bufnr('$'))
@@ -1735,6 +1737,35 @@ func Test_throw_in_BufWritePre()
   au! throwing
 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
+
 func Test_FileChangedShell_reload()
   if !has('unix')
     return