runtime(diff): Update default links (vim/vim#13776)
Problem: Current default links for `diffAdded`, `diffChanged`, and
`diffRemoved` do not address the diff nature of the filetype.
Solution: Use `DiffAdd`, `DiffChange`, and `DiffDelete`.
closes: vim/vim#1375900b470052b
Co-authored-by: Evgeni Chasnovski <evgeni.chasnovski@gmail.com>