Mercurial native migration: keep track of migrated projects
The native migration is designed to be idempotent, so that it can be run again to fix problems as we fix them.
But there is currently no way to tell that a native project was migrated from a hg-git based one. We will at some point probably want to run again the migration to fix leftover problems.
It would be problematic to run it on projects that started directly as native (useless risk, wasted resources).
Since this will require a database field, we should make it store the current Heptapod version at the time of migration, and not just be a boolean.
As more and more projects are expected to be migrated, thanks to exposition in 0.29, this should be done ASAP.