Re: [RFC PATCH 1/2] refs: make _advance() check struct repo, not flag

The answer to whether or not a ref store should refer to a certain
object store seems unresolved because a ref store is trying to do two
separate things. Perhaps it is reasonable to associate a ref database
with an object store (so that it can validate its refs), but we would
prefer to dissociate the physical ref storage layer from the object
store. (I'm paraphrasing Johnathan Nieder here, this isn't an original
thought).

Perhaps this is a question we want to resolve when considering reftable
and other ref databases.

Either adding an explicit dependency on an object store to a ref store
or dissociating it would be an improvement over what we have now, which
is an implicit dependency on the_repository's object store. Of the two,
I also prefer dissociating it. In practice, if I remember correctly, the
part that checks object existence during ref writing is the last
dependency, so if we can eliminate that without a convoluted design, I
think it's worth dissociating.