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