https://gerrit.wikimedia.org/r/c/mediawiki/core/+/447099 changes the
signature of ContentHandler::getUndoContent from <Revision, Revision,
Revision> to <Revision|Content, Revision|Content, Revision|Content[, bool]>
(with Revision soft-deprecated). This is a breaking change as
ContentHandler is meant to be subclassed, and any subclass that typehints
Revision will raise a warning at compile-time, and a fatal when called with
Content.
Searching Gerrit, Github and Google only yielded a single instance of a
subclass overriding the method (which is being fixed) and providing
backwards compatibility in a robust way does not seem feasible, so we are
changing the method signature without a deprecation period (in the sense of
subclass compatibility; with respect to calling the method with Revision,
we follow the normal deprecation process).
For more information on the MediaWiki deprecation policy see
https://www.mediawiki.org/wiki/Deprecation_policy#Removal_without_deprecati…