ui: introduce an experimental dict of exportable environment variables Care needs to be taken to prevent leaking potentially sensitive environment variables through hgweb, if template support for environment variables is to be introduced. There are a few ideas about the API for preventing accidental leaking [1]. Option 3 seems best from the POV of not needing to configure anything in the normal case. I couldn't figure out how to do that, so guard it with an experimental option for now. [1] https://www.mercurial-scm.org/pipermail/mercurial-devel/2017-January/092383.html