Well-written Vim color schemes run the command :highlight clear to
remove the highlight groups set by a previous color scheme.
Unfortunately, this also removes the internal highlight group set by
vim-exchange, effectively disabling vim-exchange's highlighting, and
running :XchangeHighlightToggle does not reenable the plugin's
highlighting.

In this commit, this is fixed by adding an autocmd to link the
_exchange_region highlight group back to ExchangeRegion when the color
scheme is changed and the highlighting is enabled.

New vim restores default highlight links, but fails to do it when there is no default link in the first place.

Without this fix, vim-exchange looses highlighting after `:colorscheme ...`

Relates to vim/vim#6970

Co-authored-by: Tom McDonald <tommcdo@gmail.com>

This reverts commit 17f1a2c, reversing
changes made to 13d3895.