This is an experimental BDB obliterate based on the deltify code. It
replaces a file's rep with an empty delta to the previous rep, thus
effectively making the file contents equal to the previous version
of the file.

This is a partial implementation of the cc1 model in notes/obliterate,
restricted to file text obliteration.  It could probably be extended
to file and directory properties, but anything that changes the tree
structure or anything that needs new node-rev-ids is not going to
work.

At present the svn_fs.h interface exists to allow testing; I haven't
really thought much about what it should look like.

- Does it work?  It passes the one regression test in fs-test.

- Is it safe to run on a live repository?  I don't know.

- Does it break younger revisions?  I don't know.  Probably not,
  because the old rep remains it the database.

- The old rep is not deleted, can it leak?  I don't know.  Could diff
  leak it?  Does diff even work properly?