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.

color: point to the global help in the example hgrc This is probably the best way to have users learn more is they need too.