Delete the old export dbs on upgrade.
Testing this an exporting to a directory with both exporttree=yes and
importtree=yes, it refused to let an interrupted export proceed after
upgrade, with "unsafe to overwrite file". An import resolved the
problem.
It will be populated automatically by the next command that needs data
from it, the same way it gets populated in a fresh clone. That may be a
little expensive, but it's a one time cost, and no slower than in a
fresh clone.
The old db is cleaned up when a new incremental fsck is started.
The incremental fsck won't pick up where the old one left off, but I
consider this a minor enough thing that it can just be documented and
won't be a problem.
The only good thing about it is it does not require a major version bump
to improve the database. That will need to happen at some point though.
Potentially very very slow in a large repository.
Ugly use of raw sql.
