Right now, we simply turn on connectivity checks in the indexer as soon
as we have access to an object database. But seeing that the
connectivity checks may incur additional overhead, we do want the user
to decide for himself whether he wants to allow those checks.
Furthermore, it might also be desirable to check connectivity in case
where no object database is given at all, e.g. in case where a fully
connected pack file is expected.

Add a flag `verify` to `git_indexer_options` to enable additional
verification checks. Also avoid to query the ODB in case none is given
to allow users to enable checks when they do not have an ODB.