New buffer "directories"

[mwtkrayer]

I added a new buffer "directories" for myself which allows browsing mail by underlying directory structure in the notmuch database directory. It is heavily based on the "namedqueries" buffer, but constructs "path:" queries. Unfortunately, the directory structure seems not to be easily deducible from the notmuch database itself, so I rely on os.walk(self.path) to construct it. I am exclusively working with "Maildir" for which this approach works fine, however, i did not test it for other storage formats.

[mjg]

You could use xapian to list all terms with prefixesP resp. XFOLDER:. On the other hand, notmuch's own shell completion simply walks the filesystem, too.