Init folder list early enough

We need the list of folders and the folder delimiter, but it was not
always retrieved early enough. E.g. when doing IMAP<->IMAP sync and the
local IMAP being readonly, we would bunk out with a mysterious error
message become repository.getsel() would still return None.

This commit fixes this error.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

Changelog.draft.rst

@@ -31,3 +31,8 @@ Changes
 
 Bug Fixes
 ---------
+
+* IMAP<->IMAP sync with a readonly local IMAP repository failed with a
+  rather mysterious "TypeError: expected a character buffer object"
+  error. Fix this my retrieving the list of folders early enough even
+  for readonly repositories.