pager: set some environment variables if they're not set Git did this already [1] [2]. We want this behavior too [3]. This provides a better default user experience (like, supporting colors) if users have things like "PAGER=less" set, which is not uncommon. The environment variables are provided by a method so extensions can override them on demand. [1]: https://github.com/git/git/blob/6a5ff7acb5965718cc7016c0ab6c601454fd7cde/pager.c#L87 [2]: https://github.com/git/git/blob/6a5ff7acb5965718cc7016c0ab6c601454fd7cde/Makefile#L1545 [3]: https://www.mercurial-scm.org/pipermail/mercurial-devel/2017-March/094780.html