Don't set foreground colour for `Diff*` groups?

[tombh]

I just installed the Diffview plugin and noticed that syntax highlighting was being overwritten by the Diff* groups. With this override I got the syntax highlighting back and tidied up the diff-changed colours:

overrides = function(c)
  return {
    DiffAdd = {
      bg = c.diff.add,
    },
    DiffChange = {},
    DiffDelete = {
      bg = c.diff.delete,
    },
    DiffText = { bg = c.diff.change },
  }
end,

Edit: added link to Diffview plugin

[luisdavim]

@tombh could you maybe open a PR for this?