Fix for MacOS issue with diffopts=internal

Stock vim on Catalina (and, apparently, Big Sur) does not support
diffopt=internal even though the default options include internal.
Thus :set diffopt+=vertical is equivalent to
:set diffopt=internal,filler,vertical which is an error because
internal is not supported. One solution is to modify your .vimrc
as follows:

if &diff
    set diffopt-=internal
    set diffopt+=vertical
endif

The issue (and fix) is described on these websites:
thoughtbot#655
https://www.micahsmith.com/blog/2019/11/fixing-vim-invalid-argument-diffopt-iwhite/

vimrc

@@ -173,6 +173,10 @@ set spellfile=$HOME/.vim-spell-en.utf-8.add
 " Autocomplete with dictionary words when spell check is on
 set complete+=kspell
 
+" Fix for MacOS issue described here:
+" https://github.com/thoughtbot/dotfiles/issues/655
+set diffopt-=internal
+
 " Always use vertical diffs
 set diffopt+=vertical