WARNING: database upgrade coming

[David Bremner]

I will fairly soon push some version of jani's patches changing folder:
matching to master. This will require an irreversible database upgrade.

Of course we believe everything is going to go smoothly, but you
probably should not run this upcoming version of notmuch on your live
mail store until you understand the implications for your workflow.

It never hurts to backup your tags before upgrading notmuch. We've never
had a data loss yet, but it _is_ software.

d