config: introduce "built-in" default configuration settings in default.d This helps providing a more consistent user experience on all platforms and with all packaging. The exact location of default.d depends on how Mercurial is installed and whether it is 'frozen'. The exact location should never be relevant to users and is intentionally not explained in details in the documentation. It will however always be next to the help and templates files. Note that setting HGRCPATH also disables these defaults. I don't know if that should be considered a bug or a feature.