RFC: Multiple filenames for email messages

[Jan Janak <jan@ryngle.com>]

The comment of _notmuch_message_set_filename says:

   XXX: We should still figure out if we think it's important to store
   multiple filenames for email messages with identical message IDs.

I have lots of such messages in my email collection, both in my local
copy of my Gmail account and also in the local copy of my company's
IMAP account.

My dream mail indexing tool should be able to apply tags automatically
based on, among other things, the name of the directory the message is
stored in. If there are multiple copies of the same message scattered
across multiple directories, I would like to apply more tags.

I assume that most tags will be applied (either manually or
automatically) after 'notmuch-new', I currently do some of it with a
simple shell script. The script does not apply tags based on directory
names yet, but it would make notmuch really flexible if we could do
that *and* if we could get access to all filenames of a particular
message.

I'd like to propose that we store all filenames for email messages in
the database, not just one per message. I'd be happy to work on it and
submit a patch if others think that this would be good to have.

  -- Jan