censor: implement censoring for revlogv2
It is a bit verbose and rough, but it works. Most of that logic can be common
for `stripping`, so we can expect more refactoring of that code to accommodate
both needs. However I wanted to keep this changesets "simple enough" and before
moving forward.
We also need to properly delete the older index/data/sidedata file, but this has
implication for streaming clone and transaction, so this will come later.
Differential Revision: https://phab.mercurial-scm.org/D10869
Run Mercurial tests with Vagrant:
$ vagrant up
$ vagrant ssh -c ./run-tests.sh