templates: adding a config() function for template customization This allows templates to be written such that users can customize them easily, or that they can be customized based on other configuration of the system. For enterprise deployments, we often have complex template aliases, and right now the only way individual users can customize those is by replacing the whole template alias (which means they won't get company-wide updates to it anymore, plus most users don't want to have to get a complex template right). With this change, they can just set a config option which feeds into our templates for common changes (e.g. whether to limit commit descriptions to the width of their terminal or not). To work around the issue of having to register the config options, I declared a dedicated section [templateconfig] for these options to be dynamically declared. They can still reference any other config option that's registered elsewhere. I only did string, bool and int at this time - list and date would add other complications with parsing the default so I'll leave that as an exercise to the reader :) Differential Revision: https://phab.mercurial-scm.org/D5959