[RFC] Implementing obliterate as a node-rev-id blacklist

I was wondering whether it would be possible to implement Julian's dd1
model of obliterate, where obliterate causes file and directory nodes
to be removed from the history, as a node-rev-id blacklist. Rather
than have obliterate modify history by rewriting previous revisions
libsvn_repos would simply add the obliterated nodes to a blacklist,
with storage provided by the fs layer, and then libsvn_repos would
query the blacklist before sending out node information. Think of it
as a path based auth system built into libsvn_repos.

One advantage of doing it this way would be that, apart from some
relatively simple fs storage/query stuff, the clever bits all happen
at the repos level and so only have to be implemented once to get both
FSFS and BDB implementations.

One disadvantage is that the querying would add some overhead to all
repos operations, although we could probably arrange for this to be
minimal on repositories that are configured not to support
obliteration.

One feature that could be viewed as either an advantage or a
disadvantage is that obliteration could even be reversible. Removing
a blacklisted node-rev-id from the blacklist would "unobliterate" it.

-- 
Philip
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