update: apply .mpdignore matches to subdirectories

Wildcard matches are directly applied to all filenames in
subdirectories without any attempt at matching relative paths.

This change is based on the following feature request:

  http://bugs.musicpd.org/view.php?id=3729

src/db/update/ExcludeList.hxx

@@ -36,15 +36,23 @@
 class Path;
 
 class ExcludeList {
+	const ExcludeList *const parent;
+
 #ifdef HAVE_CLASS_GLOB
 	std::forward_list<Glob> patterns;
 #endif
 
 public:
+	ExcludeList()
+		:parent(nullptr) {}
+
+	ExcludeList(const ExcludeList &_parent)
+		:parent(&_parent) {}
+
 	gcc_pure
 	bool IsEmpty() const {
 #ifdef HAVE_CLASS_GLOB
-		return patterns.empty();
+		return ((parent == nullptr) || parent->IsEmpty()) && patterns.empty();
 #else
 		/* not implemented */
 		return true;