Oh, better idea for write...

Pass in an optional external data handler on write.  If there is one it should
be able to provide the data for the base of the revision for diff purposes, and
it should be able to provide a cookie that will be given to the external data
handler for read.

That way the external data handler doesn't have to remember any associations
between the revision and the data.  It will be able to the revlog to hand it the
cookie which will allow it to fetch those.