Break undo history

Make undo behavior after using easymotion in operator pending mode
consistent with Vim's default behavior

Problem and how to reproduce:

1. `exec normal! "i{text}<C-[>"` or something
2. `d<Leader><Leader>s{target}`
3. type `u`

In this case, undo should restore second operation only, but it restored
first operation in addition to second one because undojoin command is
called in SetLines() function.

Solution: break undo  history by `let &undolevels = &undolevels` 
See:  :h undojoin


This commit fix this problem.