difference: has_children -> has_visible_children, and take into account comments

An earlier commit made the [+]/[-] behaviour also show/hide comments so we
should add that logic to Difference as well. Also, we use the phrase "children"
elsewhere to refer to only the _details, so use a different phrase here.

diffoscope/difference.py

@@ -115,14 +115,14 @@ class Difference(object):
                 sum(map(len, self.comments)) +
                 sum(v.size() for v in self._visuals))
 
-    def has_children(self):
+    def has_visible_children(self):
         """
-        Whether there are children.
+        Whether there are visible children.
 
         Useful for e.g. choosing whether to display [+]/[-] controls.
         """
-
-        return self._unified_diff is not None or self._details or self._visuals
+        return (self._unified_diff is not None or
+                self._comments or self._details or self._visuals)
 
     def traverse_depth(self, depth=-1):
         yield self

diffoscope/presenters/html/html.py

@@ -337,7 +337,7 @@ class HTMLPresenter(Presenter):
         print_func(u'<div class="difference">')
         try:
             print_func(u'<div class="diffheader">')
-            diffcontrol = ("diffcontrol", u'⊟') if difference.has_children() else ("diffcontrol-nochildren", u'⊡')
+            diffcontrol = ("diffcontrol", u'⊟') if difference.has_visible_children() else ("diffcontrol-nochildren", u'⊡')
             if difference.source1 == difference.source2:
                 print_func(u'<div class="%s">%s</div>' % diffcontrol)
                 print_func(u'<div><span class="source">%s</span>'