fix(inccommand): skip input() during preview #35272

Problem:
During preview, the `input` still prompts the user to enter something
that won't be used later, which could be a bit confusing.
e.g., `:s/a/\=input("")`.

Solution:
Make the input() return early during 'inccommand' preview.

src/nvim/ex_getln.c

@@ -4819,6 +4819,10 @@ void get_user_input(const typval_T *const argvars, typval_T *const rettv, const
   rettv->v_type = VAR_STRING;
   rettv->vval.v_string = NULL;
 
+  if (cmdpreview) {
+    return;
+  }
+
   const char *prompt;
   const char *defstr = "";
   typval_T *cancelreturn = NULL;

test/functional/ui/inccommand_spec.lua

@@ -2937,3 +2937,15 @@ it("'inccommand' disables preview if preview buffer can't be created #27086", fu
   ]])
   eq('nosplit', api.nvim_get_option_value('inccommand', {}))
 end)
+
+it(':substitute with inccommand, does not show prompt during preview #11940', function()
+  clear()
+  local screen = Screen.new(30, 3)
+  common_setup(screen, 'split', 'foo')
+  feed([[:s/f/\=input("sub: ")]])
+  screen:expect([[
+    oo                            |
+    {1:~                             }|
+    :s/f/\=input("sub: ")^         |
+  ]])
+end)