Update syntax highlighting in diff

Remove syntax off as it turned syntax off for the whole session,
replace with diff highlighting and keeping syntax on during diff

vimrc

@@ -31,9 +31,11 @@ set softtabstop=2
 au BufRead,BufNewFile *.ipynb setlocal filetype=json
 au BufRead,BufNewFile *.md setlocal spell
 syntax on
+
 if &diff
-        syntax off
+  highlight! link DiffText MatchParen
 endif
+au OptionSet diff highlight! link DiffText MatchParen
 
 set expandtab
 set autoindent