pager: test the 'enable' config option While poking at this option I realised it was not tested.

config: drop pager from the recommended extension The extension is deprecated, we should having exposing it to users.

config: use "churn" as an example extension "Churn" is not the useful example we have, but this is the one used in 'hg help config.extensions'. As we need something to replace the deprecated 'pager' extension in the example config, we are adding 'churn'.

discovery: prevent crash caused by prune marker having no parent data If a marker has no parent information, parents field is set to None, which caused TypeError. I think this shouldn't normally happen, but somehow I have such marker in my repo.