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

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.

pythonx/UltiSnips/snippet_manager.py

@@ -155,7 +155,7 @@ def __init__(self, expand_trigger, forward_trigger, backward_trigger):
  def jump_forwards(self):
  """Jumps to the next tabstop."""
  vim_helper.command("let g:ulti_jump_forwards_res = 1")
- vim_helper.command("let &undolevels = &undolevels")
+ vim_helper.command("let &g:undolevels = &g:undolevels")
  if not self._jump(JumpDirection.FORWARD):
  vim_helper.command("let g:ulti_jump_forwards_res = 0")
  return self._handle_failure(self.forward_trigger)
@@ -165,7 +165,7 @@ def jump_forwards(self):
  def jump_backwards(self):
  """Jumps to the previous tabstop."""
  vim_helper.command("let g:ulti_jump_backwards_res = 1")
- vim_helper.command("let &undolevels = &undolevels")
+ vim_helper.command("let &g:undolevels = &g:undolevels")
  if not self._jump(JumpDirection.BACKWARD):
  vim_helper.command("let g:ulti_jump_backwards_res = 0")
  return self._handle_failure(self.backward_trigger)
@@ -808,15 +808,15 @@ def _try_expand(self, autotrigger_only=False):
  if not snippets:
  # No snippet found
  return False
- vim_helper.command("let &undolevels = &undolevels")
+ vim_helper.command("let &g:undolevels = &g:undolevels")
  if len(snippets) == 1:
  snippet = snippets[0]
  else:
  snippet = _ask_snippets(snippets)
  if not snippet:
  return True
  self._do_snippet(snippet, before)
- vim_helper.command("let &undolevels = &undolevels")
+ vim_helper.command("let &g:undolevels = &g:undolevels")
  return True
 
  def can_expand(self, autotrigger_only=False):