Combine Diff* foreground colours with SignColumn's background.

For default sign colours.

It seems the Diff* highlight groups are generally designed for line
highlighting so they have background colours which differ from the main
background colour.  While desirable for line highlights, this isn't what
we want for the signs.

autoload/gitgutter/highlight.vim

@@ -45,14 +45,19 @@ function! gitgutter#highlight#define_highlights() abort
 
   " Highlights used by the signs.
 
+  " When they are invisible.
   execute "highlight GitGutterAddInvisible    guifg=bg guibg=" . guibg . " ctermfg=" . ctermbg . " ctermbg=" . ctermbg
   execute "highlight GitGutterChangeInvisible guifg=bg guibg=" . guibg . " ctermfg=" . ctermbg . " ctermbg=" . ctermbg
   execute "highlight GitGutterDeleteInvisible guifg=bg guibg=" . guibg . " ctermfg=" . ctermbg . " ctermbg=" . ctermbg
   highlight default link GitGutterChangeDeleteInvisible GitGutterChangeInvisible
 
-  highlight default link GitGutterAdd          DiffAdd
+  " When they are visible.
-  highlight default link GitGutterChange       DiffChange
+  " By default use Diff* foreground colors with SignColumn's background.
-  highlight default link GitGutterDelete       DiffDelete
+  for type in ['Add', 'Change', 'Delete']
+    let [guifg, ctermfg] = s:get_foreground_colors('Diff'.type)
+    execute "highlight GitGutter".type."Default guifg=".guifg." guibg=".guibg." ctermfg=".ctermfg." ctermbg=".ctermbg
+    execute "highlight default link GitGutter".type." GitGutter".type."Default"
+  endfor
   highlight default link GitGutterChangeDelete GitGutterChange
 
   " Highlights used for the whole line.
@@ -126,6 +131,21 @@ function! s:define_sign_line_highlights() abort
   endif
 endfunction
 
+function! s:get_foreground_colors(group) abort
+  redir => highlight
+  silent execute 'silent highlight ' . a:group
+  redir END
+
+  let link_matches = matchlist(highlight, 'links to \(\S\+\)')
+  if len(link_matches) > 0 " follow the link
+    return s:get_foreground_colors(link_matches[1])
+  endif
+
+  let ctermfg = s:match_highlight(highlight, 'ctermfg=\([0-9A-Za-z]\+\)')
+  let guifg   = s:match_highlight(highlight, 'guifg=\([#0-9A-Za-z]\+\)')
+  return [guifg, ctermfg]
+endfunction
+
 function! s:get_background_colors(group) abort
   redir => highlight
   silent execute 'silent highlight ' . a:group