Avoid changing 'undolevels' when creating an undo break.

Setting the 'undolevels' option to any value has the side effect of creating an undo break. UltiSnips uses the below construct to create an undo break with the goal of leaving 'undolevels' unmodified:

:let &undolevels = &undolevels

However, if a local 'undolevels' option has been set to a different value than the global option, the assignment above has the unintended effect of changing the global 'undolevels' value to the local value.

Use &g:undolevels to explicitly read and modify only the global option value, avoiding undesired option changes.

See the related pull request https://github.com/vim/vim/pull/12729 suggesting related Vim documentation changes.

Currently, UltiSnips crashes on Neovim due in part to the issue described in this pull request: https://github.com/neovim/neovim/pull/24574 (but already fixed by https://github.com/neovim/neovim/pull/24501).

To trigger this crash:

Launch Neovim:
```
nvim
```
Split the buffer into two windows:
```
wincmd v
```
Launch a substitute command:
```
:s/.*/
```
Trigger a snippet, leading to an error similar to:

An error occured. This is either a bug in UltiSnips or a bug in a
snippet definition. If you think this is a bug, please report it to
https://github.com/SirVer/ultisnips/issues/new
Please read and follow:
https://github.com/SirVer/ultisnips/blob/master/CONTRIBUTING.md#reproducing-bugs

Following is the full stack trace:
Traceback (most recent call last):
  File "/home/mike/.vim/bundle/ultisnips/pythonx/UltiSnips/err_to_scratch_buffer.py", line 47, in wrapper
    return func(self, *args, **kwds)
  File "/home/mike/.vim/bundle/ultisnips/pythonx/UltiSnips/snippet_manager.py", line 192, in expand_or_jump
    rv = self._try_expand()
  File "/home/mike/.vim/bundle/ultisnips/pythonx/UltiSnips/snippet_manager.py", line 811, in _try_expand
    vim_helper.command("let &undolevels = &undolevels")
  File "/home/mike/.vim/bundle/ultisnips/pythonx/UltiSnips/vim_helper.py", line 118, in command
    return vim.command(cmd)
  File "/opt/pynvim/lib/python3.10/site-packages/pynvim/api/nvim.py", line 287, in command
    return self.request('nvim_command', string, **kwargs)
  File "/opt/pynvim/lib/python3.10/site-packages/pynvim/api/nvim.py", line 182, in request
    res = self._session.request(name, *args, **kwargs)
  File "/opt/pynvim/lib/python3.10/site-packages/pynvim/msgpack_rpc/session.py", line 102, in request
    raise self.error_wrapper(err)
pynvim.api.common.NvimError: function ExpandSnippetOrJumpOrSkel[1]..UltiSnips#ExpandSnippetOrJump[2]..provider#python3#Call[18]..UltiSnips#ExpandSnippetOrJump, line 2: Vim(let):E474: Invalid argument

In the final line, the error Vim(let):E474: Invalid argument derives from Neovim's cmdpreview feature leaving the buffer-local 'undolevels' value set to LONG_MAX, which (on some machines) is outside the range of int that Neovim currently enforces for all options.

So in addition to properly retaining the global 'undolevels' option value, this change to UltiSnips also avoids the above UltiSnips crash on Neovim.

SirVer / ultisnips

Avoid changing 'undolevels' when creating an undo break. #1534