This is a proposed last-minute change for 0.13. It fixes the notmuch_database_get_directory API in the same way we're fixing notmuch_database_open, etc in this release. Since this is a backwards-incompatible change, it would be nice to lump it with the other API-breaking changes.
To keep the patch simple, this does not change the behavior of notmuch_database_get_directory, but it puts us in a good position to fix it in the future.