Fix undo via `<Esc>:undojoin<cr>i` (applied via `s:get_left_motion`)

While this fixes the undo chaining in general, `<Left>` is still being
used in block-wise visual mode, where exiting insert mode would also
exit the block-wise mode.

Ref: https://github.com/Raimondi/delimitMate/issues/138