Fix "getfolders() asked to get nonexisting folder" bug

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>

Changelog.draft.rst

@@ -18,3 +18,7 @@ Changes
 
 Bug Fixes
 ---------
+
+* Fix the missing folder error that occured when a new remote folder was
+  detected (IMAP<->Maildir)
+

offlineimap/repository/Maildir.py

@@ -191,3 +191,7 @@ class MaildirRepository(BaseRepository):
             self.folders = self._getfolders_scandir(self.root)
         return self.folders
 
+    def forgetfolders(self):
+        """Forgets the cached list of folders, if any.  Useful to run
+        after a sync run."""
+        self.folders = None