fix(inccommand): don't set an invalid 'undolevels' value

Problem:    Cannot break undo by setting 'undolevels' to itself in
            'inccommand' preview callback.
Solution:   Don't set an invalid 'undolevels' value.

Co-authored-by: Michael Henry <drmikehenry@drmikehenry.com>
(cherry picked from commit 7c5e7d831a)

src/nvim/ex_getln.c

@@ -2352,7 +2352,7 @@ static void cmdpreview_prepare(CpInfo *cpinfo)
       pmap_put(ptr_t)(&saved_bufs, buf, NULL);
 
       u_clearall(buf);
-      buf->b_p_ul = LONG_MAX;  // Make sure we can undo all changes
+      buf->b_p_ul = INT_MAX;  // Make sure we can undo all changes
     }
 
     CpWinInfo cp_wininfo;

test/functional/ui/inccommand_user_spec.lua

@@ -239,7 +239,8 @@ describe("'inccommand' for user commands", function()
       [1] = {background = Screen.colors.Yellow1},
       [2] = {foreground = Screen.colors.Blue1, bold = true},
       [3] = {reverse = true},
-      [4] = {reverse = true, bold = true}
+      [4] = {reverse = true, bold = true},
+      [5] = {foreground = Screen.colors.Blue},
     })
     screen:attach()
     exec_lua(setup_replace_cmd)
@@ -458,9 +459,8 @@ describe("'inccommand' for user commands", function()
     assert_alive()
   end)
 
-  it('breaking undo chain in Insert mode works properly #20248', function()
+  local function test_preview_break_undo()
     command('set inccommand=nosplit')
-    command('inoremap . .<C-G>u')
     exec_lua([[
       vim.api.nvim_create_user_command('Test', function() end, {
         nargs = 1,
@@ -490,6 +490,26 @@ describe("'inccommand' for user commands", function()
       {2:~                                       }|
       :Test a.a.a.a.^                          |
     ]])
+    feed('<C-V><Esc>u')
+    screen:expect([[
+        text on line 1                        |
+        more text on line 2                   |
+        oh no, even more text                 |
+        will the text ever stop               |
+        oh well                               |
+        did the text stop                     |
+        why won't it stop                     |
+        make the text stop                    |
+      a.a.a.                                  |
+      {2:~                                       }|
+      {2:~                                       }|
+      {2:~                                       }|
+      {2:~                                       }|
+      {2:~                                       }|
+      {2:~                                       }|
+      {2:~                                       }|
+      :Test a.a.a.a.{5:^[}u^                       |
+    ]])
     feed('<Esc>')
     screen:expect([[
         text on line 1                        |
@@ -510,6 +530,18 @@ describe("'inccommand' for user commands", function()
       {2:~                                       }|
                                               |
     ]])
+  end
+
+  describe('breaking undo chain in Insert mode works properly', function()
+    it('when using i_CTRL-G_u #20248', function()
+      command('inoremap . .<C-G>u')
+      test_preview_break_undo()
+    end)
+
+    it('when setting &l:undolevels to itself #24575', function()
+      command('inoremap . .<Cmd>let &l:undolevels = &l:undolevels<CR>')
+      test_preview_break_undo()
+    end)
   end)
 end)