vim-patch:9.0.1299: change for triggering incsearch not sufficiently tested

Problem:    Change for triggering incsearch not sufficiently tested.
Solution:   Add a test case.  Simplify the code. (closes vim/vim#11971)

412e0e4ed9

src/nvim/ex_getln.c

@@ -1626,9 +1626,9 @@ static int command_line_insert_reg(CommandLineState *s)
   ccline.special_char = NUL;
   redrawcmd();
 
-  // The text has been stuffed, the command line didn't change yet, but it
-  // will change soon.  The caller must take care of it.
-  return literally ? CMDLINE_NOT_CHANGED : CMDLINE_CHANGED;
+  // With "literally": the command line has already changed.
+  // Else: the text has been stuffed, but the command line didn't change yet.
+  return literally ? CMDLINE_CHANGED : CMDLINE_NOT_CHANGED;
 }
 
 /// Handle the Left and Right mouse clicks in the command-line mode.
@@ -1862,9 +1862,8 @@ static int command_line_handle_key(CommandLineState *s)
     switch (command_line_insert_reg(s)) {
     case GOTO_NORMAL_MODE:
       return 0;  // back to cmd mode
-    case CMDLINE_NOT_CHANGED:
-      s->is_state.incsearch_postponed = true;
-      FALLTHROUGH;
+    case CMDLINE_CHANGED:
+      return command_line_changed(s);
     default:
       return command_line_not_changed(s);
     }

src/nvim/testdir/test_autocmd.vim

@@ -1897,17 +1897,15 @@ func Test_Cmdline()
   call assert_equal(':', g:entered)
   au! CmdlineChanged
 
+  autocmd CmdlineChanged : let g:log += [getcmdline()]
+
   let g:log = []
   cnoremap <F1> <Cmd>call setcmdline('ls')<CR>
-  autocmd CmdlineChanged : let g:log += [getcmdline()]
   call feedkeys(":\<F1>", 'xt')
   call assert_equal(['ls'], g:log)
-  unlet g:log
-  au! CmdlineChanged
   cunmap <F1>
 
   let g:log = []
-  autocmd CmdlineChanged : let g:log += [getcmdline()]
   call feedkeys(":sign \<Tab>\<Tab>\<C-N>\<C-P>\<S-Tab>\<S-Tab>\<Esc>", 'xt')
   call assert_equal([
         \ 's',
@@ -1938,6 +1936,22 @@ func Test_Cmdline()
         \ 'sign unplace',
         \ ], g:log)
   set wildmenu& wildoptions&
+
+  let g:log = []
+  let @r = 'abc'
+  call feedkeys(":0\<C-R>r1\<C-R>\<C-O>r2\<C-R>\<C-R>r3\<Esc>", 'xt')
+  call assert_equal([
+        \ '0',
+        \ '0a',
+        \ '0ab',
+        \ '0abc',
+        \ '0abc1',
+        \ '0abc1abc',
+        \ '0abc1abc2',
+        \ '0abc1abc2abc',
+        \ '0abc1abc2abc3',
+        \ ], g:log)
+
   unlet g:log
   au! CmdlineChanged