This source driver allows vcp to read a set of revisions from
a metadata database written by VCP::Dest::metadb.
NOTE: passing a reference in the options array to new:
$s = VCP::Source::metadb->new( $spec, [ $real_source ] );
sets the metadbs source to point to the real repository it should
get revisions from. This should only be necessary for a transfer
operation, as filtering does not need to access the source repository.