New option to only work with subscribed folders

Patch from Sean Finney, slightly modified by John Goerzen to change
name of config file option

fixes deb#499588

offlineimap.conf

@@ -212,6 +212,10 @@ type = Maildir
 
 localfolders = ~/Test
 
+# Specify whether to process all mail folders on the server, or only
+# those listed as "subscribed".
+subscribedonly = no
+
 # You can specify the "path separator character" used for your Maildir
 # folders.  This is inserted in-between the components of the tree.
 # It defaults to ".".  If you want your Maildir folders to be nested,

offlineimap/repository/IMAP.py

@@ -192,8 +192,13 @@ class IMAPRepository(BaseRepository):
             return self.folders
         retval = []
         imapobj = self.imapserver.acquireconnection()
+        # check whether to list all folders, or subscribed only
+        listfunction = imapobj.list
+        if self.config.has_option(self.getsection(), 'subscribedonly'):
+          if self.getconf('subscribedonly') == "yes":
+            listfunction = imapobj.lsub
         try:
-            listresult = imapobj.list(directory = self.imapserver.reference)[1]
+            listresult = listfunction(directory = self.imapserver.reference)[1]
         finally:
             self.imapserver.releaseconnection(imapobj)
         for string in listresult: