When a new remote folder was detected, we tried to create the folder
locally on the Maildir and called repository.forgetfolders() to force a
new scanning of the Maildir. However, that implementation used the
inherited base function that did nothing. We simply needed to implement
forgetfolders() to set self.folder=None, so we would force a new read in
of the updated local folder structure.
Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>