vim-patch:8.2.2870: CmdlineChange event triggered twice for CTRL-R (#21361)

Problem:    CmdlineChange event triggered twice for CTRL-R.
Solution:   Return CMDLINE_NOT_CHANGED from cmdline_insert_reg().
            (closes vim/vim#8219)

796139ae3a

Cherry-pick Test_cmdline_map_cmdlineChanged() from patch 8.2.2851.

Co-authored-by: Bram Moolenaar <Bram@vim.org>

src/nvim/ex_getln.c

@@ -1609,7 +1609,8 @@ static int command_line_insert_reg(CommandLineState *s)
   ccline.special_char = NUL;
   redrawcmd();
 
-  return CMDLINE_CHANGED;
+  // The text has been stuffed, the command line didn't change yet.
+  return CMDLINE_NOT_CHANGED;
 }
 
 /// Handle the Left and Right mouse clicks in the command-line mode.

src/nvim/testdir/test_cmdline.vim

@@ -2235,6 +2235,30 @@ func Test_recalling_cmdline()
   cunmap <Plug>(save-cmdline)
 endfunc
 
+func Test_cmd_map_cmdlineChanged()
+  let g:log = []
+  cnoremap <F1> l<Cmd><CR>s
+  augroup test
+    autocmd!
+    autocmd CmdlineChanged : let g:log += [getcmdline()]
+  augroup END
+
+  call feedkeys(":\<F1>\<CR>", 'xt')
+  call assert_equal(['l', 'ls'], g:log)
+
+  let @b = 'b'
+  cnoremap <F1> a<C-R>b
+  let g:log = []
+  call feedkeys(":\<F1>\<CR>", 'xt')
+  call assert_equal(['a', 'ab'], g:log)
+
+  unlet g:log
+  cunmap <F1>
+  augroup test
+    autocmd!
+  augroup END
+endfunc
+
 " Test for the 'suffixes' option
 func Test_suffixes_opt()
   call writefile([], 'Xfile')

test/functional/autocmd/cmdline_spec.lua

@@ -223,7 +223,6 @@ describe('cmdline autocommands', function()
     eq({'notification', 'CmdlineChanged', {{cmdtype='=', cmdlevel=2}, "1+1"}}, next_msg())
     feed('<cr>')
     eq({'notification', 'CmdlineLeave', {{cmdtype='=', cmdlevel=2, abort=false}}}, next_msg())
-    eq({'notification', 'CmdlineChanged', {{cmdtype=':', cmdlevel=1}, "let x = "}}, next_msg())
     eq({'notification', 'CmdlineChanged', {{cmdtype=':', cmdlevel=1}, "let x = 2"}}, next_msg())
     feed('<cr>')
     eq({'notification', 'CmdlineLeave', {{cmdtype=':', cmdlevel=1, abort=false}}}, next_msg())