over: store xref3 data in over.sqlite3

over: store xref3 data in over.sqlite3

We may not end up storing xref3 data in Xapian, actually.
This will make indexlevel=basic possible, and along with
--sequential-shard indexing support for slow storage.

Making oidmap a separate table seems unnecessary, too, so
fold it into the xref3 table since it's unlikely a git blob
will be responsible for multiple xref3 rows.