directory: added directory_is_root()

directory_is_root() is cheaper than
isRootDirectory(directory_get_path()).

src/directory.h

@@ -70,6 +70,15 @@ directory_get_path(const struct directory *directory)
 	return directory->path;
 }
 
+/**
+ * Is this the root directory of the music database?
+ */
+static inline bool
+directory_is_root(const struct directory *directory)
+{
+	return directory->parent == NULL;
+}
+
 /**
  * Returns the base name of the directory.
  */