Adds a very basic replica of ui.plain().
- Group Reviewers
- rHG38deb65d4441: rhg: add ui.plain() and check it before showing relative paths in status
This can be spelled env::var("HGPLAIN").is_ok().
However env::var has two error cases. If we only want to check for the presence of a variable regardless of its Unicode validity, env::var_os("HGPLAIN").is_some() should be used instead.