revlog: move censoring code in a dedicated module This code is quite specific and we are about to add more of it for revlog-v2 (and other derived version). So we move this code in a dedicated module in `mercurial/revlogutils/`. This looks like a good fit. The diff is huge because I used `hg copy` to create the new file so that we keep the history of the censor code. Differential Revision: https://phab.mercurial-scm.org/D10789