vim: Use gitgutter's foldtext()

Use `gitgutter`'s `foldtext()` to display an indicator if a fold
contains lines that have been changed.
This is necessary as {,neo}vim does not display signs next to folded
lines.

.config/vim/vimrc.d/30-settings.vim

@@ -76,6 +76,11 @@ set iskeyword+=-
 " Do not automatically insert <EOL> at EOF if missing
 set nofixendofline
 
+if (exists('g:loaded_gitgutter'))
+	" Augment the default `foldtext()` with an indicator whether the folded
+	" lines have been changed.
+	set foldtext=gitgutter#fold#foldtext()
+endif
 
 " Netrw