Database version 19 on master branch

[Mike Shal]

Hi all,

Just a heads-up that if you are using tup from the master branch, the latest batch of commits includes an upgrade to database version 19. This adds an mtime_ns field to give nanosecond granularity on timestamps, which greatly helps tup detect differences when scripting.

The mtime_ns field will get populated the next time you run tup, but it will cause tup to think every file is out-of-date and rebuild everything after the upgrade. If you notice any timestamp issues after that please create an issue.

Thanks!

-Mike