Previously, getfolder() would always construct new MaildirFolder()
objects, independent of whether the folder exists or not. Improve the
function to:
1) Scan and cache the folders if not already done
2) Return the same cached object if we ask for the same foldername twice
3) Reduce a tiny bit of code duplication
This is important because we handle stuff like folderfilter in the
scandir function and if we discard the scanned dir and create a new
object on folderget(), we will lose the folderfilter information.
Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
792243c78f