filecache: create an entry in _filecache when __set__ is called for a missing one Preserve the invariant that if P is a filecached property on X then P in X.__dict__ => P in X._filecache. Previously, it was possible for a filecached property to become out of sync with the filesystem if it was set before getting it first, since the initial filecacheentry was created in __get__. Old behaviour: repo.prop = x repo.invalidate() # prop has no entry in _filecache, it's not removed # from __dict__ repo.prop # returns x like before without checking with the # filesystem New: repo.prop = x # an empty entry is created in _filecache repo.invalidate() # prop is removed from __dict__ repo.prop # recreates prop