Handle common user errors when adding an existing torrent

[charles]

It seems fairly common for users to add preexisting torrents with the wrong directory information -- for example, if the torrent's toplevel folder is named "torrenttop" and it's saved in "/home/user", users will often specify /home/user/torrenttop instead of /home/user as the directory to save the torrent in. That's understandable, and jarring when they try that and the contents of their torrent can't be found.

In cases where the torrent's top-level directory is the same as the destination's bottom directory, and nothing can be found in the user-specified directory, we should try moving up to the parent directory and retrying.

[charles]

fixed in trunk by r11112