In response to mps's:
"Currently fixing the renaming issue would present a layering
violation. That is, individual revlogs have no knowledge of any other
revlog. So when we ask a revlog to retrieve version <x> of some file,
it has to have all the data internally."

Actually we can store in revlog a reference to generic external object,
identified by some kind of "url" and (maybe) hash
Initially it can be used to implement renames and copies but it can evolve
into some kind of super svn:external facility later (like hg repo which
retrieves file directrly form extrenal svn or git)