Add highlight for ft-diff

Fix highlighting for the diff filetype.  By default, syntax/diff.vim
uses highlights for `Identifier` and `Special` for added and removed
lines:

    hi def link diffAdded		Identifier
    hi def link diffRemoved		Special

As these are both highlighted in white in monokai.nvim, it makes diff
highlighting display everything as plain white.

Fix this by adding explicit highlights for diffAdded and diffRemoved in
appropriate colors.

Author: Rahix

lua/monokai.lua

@@ -238,6 +238,12 @@ M.load_syntax = function(palette)
             fg = palette.black,
             bg = palette.aqua,
         },
+        diffAdded = {
+            fg = palette.green,
+        },
+        diffRemoved = {
+            fg = palette.pink,
+        },
         Folded = {
             fg = palette.grey,
             bg = palette.base3,