* src/filetree.cc (FileTree::prev_match_file): As it turns out
keeping track of the parent iterator does have some merit.  While
there was nothing wrong with the previous change, I discovered
that the parent's children can be indexed in order to obtain an
iterator pointing to the previous row.  At least for deep trees
this is likely to be faster than resolving the whole path via
Gtk::TreeModel::get_iter().