The censor system allows retroactively removing content from
    files. Actually censoring a node requires using the censor extension,
    but the functionality for handling censored nodes is partially in core.
    Censored nodes in a filelog have the flag ``REVIDX_ISCENSORED`` set,
    and the contents of the censored node are replaced with a censor
    tombstone. For historical reasons, the tombstone is packed in the
    filelog metadata field ``censored``. This allows censored nodes to be
    (mostly) safely transmitted through old formats like changegroup
    versions 1 and 2. When using changegroup formats older than 3, the
    receiver is required to re-add the ``REVIDX_ISCENSORED`` flag when
    storing the revision. This depends on the ``censored`` metadata key
    never being used for anything other than censoring revisions, which is
    true as of January 2017. Note that the revlog flag is the
    authoritative marker of a censored node: the tombstone should only be
    consulted when looking for a reason a node was censored or when revlog
    flags are unavailable as mentioned above.
    The tombstone data is a free-form string. It's expected that users of
    censor will want to record the reason for censoring a node in the
    tombstone. Censored nodes must be able to fit in the size of the
    content being censored.