folder/IMAP: savemessage() should just save flags if uid already exists

As the LocalStatus and UIDMap backend already did: If the uid already
exists for savemessage(), only modify the flags and don't append a new
message.

We don't invoke savemessage() on messages that already exist in our sync
logic, so this has no change on our current behavior. But it makes
backends befave more consistent with each other.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/folder/IMAP.py

@@ -394,6 +394,11 @@ class IMAPFolder(BaseFolder):
                   server. If the folder is read-only it will return 0."""
         self.ui.debug('imap', 'savemessage: called')
 
+        # already have it, just save modified flags
+        if uid > 0 and uid in self.messagelist:
+            self.savemessageflags(uid, flags)
+            return uid
+
         try:
             imapobj = self.imapserver.acquireconnection()