[rdiff-backup-users] how to show directories only in backup at given time

[Maciej Franecki]

hello.
im working on backup system for the user's Maildir. As far as we are using imap, all the users all able to make their own directories for mails in Maildir. 
I wonder if somehow is it possible to use --list-at-time option to show only directories? 
To see all the files with the directories is not really interesting in my case. Also, it is not in the format of showing rights; so grep ^d unfortunately is not the solution also.

To be sure about the result which im looking for let me present small example.
Let's assume that user mary has got three directories; cur, new, priv.
cur&new has some files inside, priv is empty.

root@:~# rdiff-backup --list-at-time now n::/tests/mary
mary
mary/b.file
mary/b.file2
mary/b.file3
mary/cur
mary/cur/reasqdsqdsq
mary/mail.mary
mary/mail5
mary/new
mary/new/sqdsqdsqs
mary/priv
root@:~# 

I would like to obtain result like:
mary/cur
mary/new
mary/priv


Thank you in advance,
Mac.