Bug: bea/169

This is an outgrowth of bea/110/bd1:
> All of this makes me wonder how much we care about being able to
> see bug diffs for any repository format older than the current one.
> I think that we don't really care ;).  After all, the on-disk
> format should settle down as BE matures :p.  When you _do_ want
> to see the long-term history of a particular bug, there's always
>   bzr log .be/123/bugs/456/values
> or the equivalent for your VCS.  If access to the raw log ends
> up being important, it should be very easy to add
>   libbe.storage.base.VersionedStorage.log(id)
>   libbe.command.log

Access to the (parsed) logs will be important for pretty-printing
bugdir/bug/comment change logs.  Since we do version the bug
repository, users will expect us to be able to list the history for
any particular item (e.g. for "last activity" timestamps, automatic
reminder emails, whatever).  While it does not necessarily need to be
able to delve into old storage formats, it does need to get
implemented.  It's probably worth encapsulating changes in something
like a list of Diff() objects, although it might be worth linking
along bug lines, etc., like VCS annotation.