censor: add censor command to hgext with basic client-side tests The censor command is a core extension which can replace the contents of a historical file revision with a censor "tombstone" which can be exchanged with older clients in place of the real revision data. The command rewrites the filelog by copying revision-by-revision. Care must be taken to expand the fulltext of the children of the censored revision before copying them to the new filelog; they might be stored as deltas against the uncensored revision, and those deltas will be invalidated. For more background on the censorship feature design, see: http://mercurial.selenic.com/wiki/CensorPlan