Suppress vm-auto-folder-alist when it points to the same folder

The variable vm-auto-folder-alist is used to suggest a folder in which to save a message. vm-virtual-auto-folder-alist also serves this purpose.

If the suggested folder is the same as the current folder (i.e., the folder in which the message resides), it doesn't make sense for VM to suggest it. In this case, other heuristics should be used such as the last saved folder etc.