Ignore lower-case custom Maildir flags

Do not read in custom maildir flags, or we would try to sync them over
the wire. The next step will be to merge flag writes with existing
custom flags, so we don't lose information.

The long term goal will be to attempt to sync flags to the other side,
of course.

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

offlineimap/folder/Maildir.py

@@ -138,7 +138,9 @@ class MaildirFolder(BaseFolder):
                 uid = long(uidmatch.group(1))
         flagmatch = self.re_flagmatch.search(filename)
         if flagmatch:
-            flags = set(flagmatch.group(1))
+            # Filter out all lowercase (custom maildir) flags. We don't
+            # handle them yet.
+            flags = set((c for c in flagmatch.group(1) if not c.islower()))
         return prefix, uid, fmd5, flags
 
     def _scanfolder(self):