Treat cleared highlight groups as not set

Setting a colorscheme often clears all existing highlight groups.  When
the plugin subsequently sets up its colours, it sees that the GitGutter*
groups exist and leaves them alone - even though they have been cleared.

This change means cleared groups do get their colours set up again.

See #727.

autoload/gitgutter/highlight.vim

@@ -77,7 +77,7 @@ function! gitgutter#highlight#define_highlights() abort
 
   " When they are visible.
   for type in ["Add", "Change", "Delete"]
-    if hlexists("GitGutter".type)
+    if hlexists("GitGutter".type) && s:get_foreground_colors('Diff'.type) != ['NONE', 'NONE']
       if g:gitgutter_set_sign_backgrounds
         execute "highlight GitGutter".type." guibg=".guibg." ctermbg=".ctermbg
       endif